CoreCLR runtime tests + Mono on the x64 iOS simulator (#43954)

CoreCLR runtime tests + Mono on the x64 iOS simulator (#43954)

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.