CoreCLR runtime tests + Mono on the x64 iOS simulator

[imhameed]

This creates another runtime-staging lane, named "Build iOSSimulator x64
Release AllSubsets_Mono_RuntimeTests", that will eventually run the runtime
test suite against Mono's non-LLVM JIT on the iOS simulator on amd64 hosts.
Failing tests are added to the exclusion lists in issues.targets.

The tests aren't set to run yet, because they currently take a very long time
to execute.

AppleAppBuilder no longer requires a MainLibraryFileName. If omitted, one
must be supplied when the app bundle is launched via an environment variable
named MONO_APPLE_APP_ENTRY_POINT_LIB_NAME. The generated apps also accept
another environment variable named MONO_APPLE_APP_ASSEMBLY_LOAD_PREFIX, which
is a hack used to allow assembly lookup to proceed in a nested app-relative
subdirectory before falling back to the root of the app bundle. This is
necessary because app bundles contain multiple individual test assemblies, and
these assemblies sometimes have dependencies with names that collide with the
dependencies of other test assemblies inside the bundle.

[ghost]

Tagging subscribers to this area: @directhex
See info in area-owners.md if you want to be subscribed.

[imhameed]

Current status: can build apps containing entire runtime test categories. Can also read stdout from instrumented native app launcher code from xcrun simctl launch --console-pty. Need to figure out why the tests don't seem to actually run (despite the runtime superficially launching correctly).

[imhameed]

Haven't spent much time on this this week; have mostly been occupied with arm64 simd.

[imhameed]

I got the tests running in the simulator. I also tried getting these running on-device, but that will require a bit more work, and won't be a part of this PR.

[SamMonoRT]

No progress made in last few days, will pick up this work next week.

[SamMonoRT]

@imhameed working on this item this week

[imhameed]

baseservices_mono.app/runningmono.dll passed locally:

XHarness command issued: apple run --app=baseservices_mono.app --output-directory=out --targets=ios-simulator-64 --mlaunch=/Library/Frameworks/Xamarin.iOS.framework/Versions/14.10.0.4/bin/mlaunch --set-env=MONO_APPLE_APP_ENTRY_POINT_LIB_NAME=runningmono.dll --expected-exit-code=100
info: Preparing run for ios-simulator-64
info: Getting app bundle information..
info: Starting application 'baseservices_mono' on ios-simulator-64
info: App run ended with 100
info: Application has finished with exit code: 100 (as expected)
XHarness exit code: 0

[imhameed]

@fanyang-mono @premun I'm going to merge this PR. I've disabled sending the tests to Helix, so this PR won't result in increased use of the OSX.1015.Amd64.Open pool. Followup work (like using CustomCommands, or completing the transition to xharness apple just-run, or reducing the end-to-end runtime of a fully-enabled iOSSimulator runtime test lane) will build on the work done in this PR.

[akoeplinger]

LGTM