[2018-04] unbreak interp-only mode in Xamarin.iOS

[lewurm]

Context: dotnet/macios#3856 (comment)

This PR

• backports [interp] fullAOT mixed mode #8838 (already exists on master and 2018-06)
• fixes an AOT compiler regression for interp support due to 67e995c (this needs to be forward ported to master and 2018-06)
• adding two workarounds to force interp-only execution. This is fine for this branch, because we aren't there yet regarding mixed mode execution, but I need to work on a better solution for master/2018-06.

Once approved, I'm going to propagate fixes to the other branches.

[vargaz]

Why is this needed ?

[lewurm]

With mixed mode logic we try to load AOT methods first; the AOT runtime tries to initialize methods from the AOT image, but fails because the requested method isn't available in the AOT image. Failing gracefully we allow the mini-runtime to try another execution engine (i.e. interp). Here's the native stack trace:

(lldb) mbt
* thread #1
    frame #0: 0x000000018173d2ec libsystem_kernel.dylib`__pthread_kill + 8
    frame #1: 0x00000001818de288 libsystem_pthread.dylib`pthread_kill$VARIANT$mp + 376
    frame #2: 0x00000001816abd0c libsystem_c.dylib`abort + 140
    frame #3: 0x0000000100baa584 nsarrayfilter`mono_log_write_asl(log_domain=0x0000000000000000, level=G_LOG_LEVEL_ERROR, hdr=0, message="* Assertion at ../../../../../mono/mini/aot-runtime.c:4333, condition `got' not met\n") at mono-log-darwin.c:41
    frame #4: 0x0000000100bab16c nsarrayfilter`structured_log_adapter(log_domain=0x0000000000000000, log_level=G_LOG_LEVEL_ERROR, message="* Assertion at ../../../../../mono/mini/aot-runtime.c:4333, condition `got' not met\n", user_data=0x0000000000000000) at mono-logger.c:458
    frame #5: 0x0000000100bc32b4 nsarrayfilter`monoeg_g_logv(log_domain=0x0000000000000000, log_level=G_LOG_LEVEL_ERROR, format="* Assertion at %s:%d, condition `%s' not met\n", args="��") at goutput.c:115
    frame #6: 0x0000000100bc33e0 nsarrayfilter`monoeg_assertion_message(format="* Assertion at %s:%d, condition `%s' not met\n") at goutput.c:146
  * frame #7: 0x000000010099c068 nsarrayfilter`init_method(amodule=0x000000010180b400, method_index=1034, method=0x000000010181a128, init_class=0x0000000000000000, context=0x0000000000000000, error=0x000000016f492eb0) at aot-runtime.c:4333
    frame #8: 0x00000001009939c8 nsarrayfilter`load_method(domain=0x0000000100f009a0, amodule=0x000000010180b400, image=0x0000000101806000, method=0x000000010181a128, token=100664331, method_index=1034, error=0x000000016f492eb0) at aot-runtime.c:4056
    frame #9: 0x00000001009933ec nsarrayfilter`mono_aot_get_method(domain=0x0000000100f009a0, method=0x000000010181a128, error=0x000000016f492eb0) at aot-runtime.c:4767
    frame #10: 0x00000001009e2b9c nsarrayfilter`mono_jit_compile_method_with_opt(method=0x000000010181a128, opt=374417919, jit_only=0, error=0x000000016f492eb0) at mini-runtime.c:2343
    frame #11: 0x00000001009e25c0 nsarrayfilter`mono_jit_compile_method(method=0x000000010181a128, error=0x000000016f492eb0) at mini-runtime.c:2440
    frame #12: 0x00000001009e565c nsarrayfilter`mono_jit_runtime_invoke(method=0x000000010181a128, obj=0x0000000101200048, params=0x000000016f492dd8, exc=0x0000000000000000, error=0x000000016f492eb0) at mini-runtime.c:2954
    frame #13: 0x0000000100aa62f4 nsarrayfilter`do_runtime_invoke(method=0x000000010181a128, obj=0x0000000101200048, params=0x000000016f492dd8, exc=0x0000000000000000, error=0x000000016f492eb0) at object.c:2942
    frame #14: 0x0000000100aa1018 nsarrayfilter`mono_runtime_invoke_checked(method=0x000000010181a128, obj=0x0000000101200048, params=0x000000016f492dd8, error=0x000000016f492eb0) at object.c:3095
    frame #15: 0x0000000100a3a968 nsarrayfilter`create_exception_two_strings(klass=0x0000000101819990, a1=0x0000000101200018, a2=0x0000000000000000, error=0x000000016f492eb0) at exception.c:222
    frame #16: 0x0000000100a3a780 nsarrayfilter`mono_exception_from_name_two_strings_checked(image=0x0000000101806000, name_space="System", name="OutOfMemoryException", a1=0x0000000101200018, a2=0x0000000000000000, error=0x000000016f492eb0) at exception.c:278
    frame #17: 0x00000001009f6c60 nsarrayfilter`create_domain_objects(domain=0x0000000100f009a0) at appdomain.c:209
    frame #18: 0x00000001009f6250 nsarrayfilter`mono_runtime_init_checked(domain=0x0000000100f009a0, start_cb=(nsarrayfilter`mono_thread_start_cb at mini-runtime.c:949), attach_cb=(nsarrayfilter`mono_thread_attach_cb at mini-runtime.c:972), error=0x000000016f493158) at appdomain.c:304
    frame #19: 0x00000001009e4f60 nsarrayfilter`mini_init(filename="MonoTouch", runtime_version="mobile") at mini-runtime.c:4363
    frame #20: 0x00000001009bf1a8 nsarrayfilter`mono_jit_init_version(domain_name="MonoTouch", runtime_version="mobile") at driver.g.c:2590
    frame #21: 0x0000000100bd16a8 nsarrayfilter`::xamarin_main(argc=<unavailable>, argv=<unavailable>, launch_mode=<unavailable>) at monotouch-main.m:421 [opt]
    frame #22: 0x0000000100971d88 nsarrayfilter`main(argc=1, argv=0x000000016f493b38) at main.m:45
    frame #23: 0x000000018160dfc0 libdyld.dylib`start + 4
(lldb) p amodule->aot_name
(char *) $6 = 0x00000001c0017a00 "mscorlib"
(lldb) p method->name
(const char *) $4 = 0x000000010131ff0d ".ctor"

However, for 2018-04, with the workarounds in 8ae79f2 this wouldn't be needed anymore. But we are going to need it in master (#9295 )