[xharness] Add ARM64 variations for macOS/.NET using both MonoVM and CoreCLR.

[rolfbjarne]

The CoreCLR version gets to the exact same place for ARM64 as the x86_64 version (good!).

The MonoVM version crashes at launch with:

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libsystem_platform.dylib      	0x0000000190fbf6c8 _platform_memset_pattern16 + 296
1   libcoreclr.dylib              	0x00000001035a49a0 mono_arch_create_sdb_trampoline + 76
2   libcoreclr.dylib              	0x000000010356b6ac mini_get_breakpoint_trampoline + 80
3   libcoreclr.dylib              	0x0000000103595908 mono_arch_init + 48
4   libcoreclr.dylib              	0x00000001034db268 mini_init + 580
5   libcoreclr.dylib              	0x000000010352eae8 mono_jit_init_version + 20
6   com.xamarin.monotouch-test    	0x0000000102ca0498 xamarin_bridge_initialize + 192 (monovm-bridge.m:84)
7   com.xamarin.monotouch-test    	0x0000000102ca5de8 xamarin_main + 1332 (monotouch-main.m:408)

This needs further investigation (the curious part is that this doesn't happen with the legacy Xamarin.Mac version).

This is due to: dotnet/runtime#52635

[vs-mobiletools-engineering-service2]

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Pipeline on Agent XAMBOT-1096.BigSur'
Merge 15051d1 into 82f8ca8

[spouliot]

@mandel-macaque there's something wrong in the file check wrt tests enabled. If xharness is modified then we should run all (non device) tests.

monotouch-test (Succeeded; Ignored)
-Mac [dotnet] (Succeeded; Ignored)
+Debug (ARM64) [dotnet] (Ignored)
+Debug (CoreCLR) [dotnet] (Ignored)
+Debug (CoreCLR, ARM64) [dotnet] (Ignored)
+Debug [dotnet] (Succeeded)

^ hmm... maybe not (in this case) since they would not run properly yet (and the dotnet variation was run).

Still any change in xharness is an unknown (when the decision to run which tests is made) so all tests should be run.