[Runtime] ARM64 trampolines crashed with the fstack-protector-strong flag enabled.

[mandel-macaque]

The trampolines crash as stated in https://github.com/xamarin/maccore/issues/2072

Looks like the code of the trampolines is stepping on the canary added by the compiler making the tests to crash. PR #7529 removes the flag, so that master is in a release state. But we need to debug which of the calls is writing on the canary and make sure it does the correct thing and does not step over the canary that is found at the end of it.

[mandel-macaque]

The crash occurs when the marshal method gets the following input:

monotouchtestmarshal_return_value(context=0x000000016f857570, type="{Sddddd=ddddd}", size=40, vvalue=0x00000001071c8828, mtype=0x000000014633dbd8, retain=false, method=0x00000001464e1de8, desc=0x00000002805669a0, exception_gchandle=0x000000016f85750c) at trampolines-arm64.m:0`

The trampoline invocation is:

monotouchtest::xamarin_invoke_trampoline(type=Tramp_Default, self=0x000000028026c010, sel="Test_dddddStruct", iterator=(monotouchtestparam_iter_next(IteratorAction, void*, char const*, unsigned long, void*, unsigned int*) at trampolines-arm64.m:140), marshal_return_value=(monotouchtestmarshal_return_value(void*, char const*, unsigned long, void*, _MonoType*, bool, _MonoMethod*, MethodDescription*, unsigned int*)`