[darwin][compiler-rt] build libclang_rt.<os>sim.a Apple Silicon slice, if SDK supports it

[arphaman]

Summary

This change lets compiler-rt build the libclang_rt.<os>sim.a library with the arm64
Apple Silicon simulator slice by default, iff the relevant simulator SDK advertises
support for the arm64 arch in the SDKSettings.plist file.

[arphaman]

don't check if the SDK isn't present.

[steven_wu]

LGTM

compiler-rt/cmake/builtin-config-ix.cmake

if(APPLE)
   # Add support for arm64 macOS if available in SDK.
   foreach(arch ${ARM64})
     sdk_has_arch_support(${DARWIN_osx_SYSROOT} macosx ${arch} MACOS_ARM_SUPPORT)
     if (MACOS_ARM_SUPPORT)
      list(APPEND DARWIN_osx_BUILTIN_ALL_POSSIBLE_ARCHS ${arch})
     endif()
   endforeach(arch)
 
+  function(set_simulator_arches all_possible_archs os sdkname)
+    set(simulator_arches ${X86} ${X86_64})
+    # Add additional slices for Apple Silicon simulator targets, if they're
+    # supported by the SDK.
+    if(NOT "${DARWIN_${os}sim_SYSROOT}" STREQUAL "")
+      foreach(arch ${ARM64})
+        sdk_has_arch_support(${DARWIN_${os}sim_SYSROOT} ${sdkname} ${arch} SIM_ARM_SUPPORT)
+        if (SIM_ARM_SUPPORT)
+          list(APPEND simulator_arches ${arch})
+        endif()
+      endforeach()
+    endif()
+    set("${all_possible_archs}" ${simulator_arches} PARENT_SCOPE)
+  endfunction()
+
   if(COMPILER_RT_ENABLE_IOS)
     list(APPEND DARWIN_EMBEDDED_PLATFORMS ios)
     set(DARWIN_ios_MIN_VER_FLAG -miphoneos-version-min)
     set(DARWIN_ios_BUILTIN_MIN_VER 6.0)
     set(DARWIN_ios_BUILTIN_MIN_VER_FLAG
       ${DARWIN_ios_MIN_VER_FLAG}=${DARWIN_ios_BUILTIN_MIN_VER})
     set(DARWIN_ios_BUILTIN_ALL_POSSIBLE_ARCHS ${ARM64} ${ARM32})
-    set(DARWIN_iossim_BUILTIN_ALL_POSSIBLE_ARCHS ${X86} ${X86_64})
+    set_simulator_arches(DARWIN_iossim_BUILTIN_ALL_POSSIBLE_ARCHS ios iphonesimulator)
   endif()
   if(COMPILER_RT_ENABLE_WATCHOS)
     list(APPEND DARWIN_EMBEDDED_PLATFORMS watchos)
     set(DARWIN_watchos_MIN_VER_FLAG -mwatchos-version-min)
     set(DARWIN_watchos_BUILTIN_MIN_VER 2.0)
     set(DARWIN_watchos_BUILTIN_MIN_VER_FLAG
       ${DARWIN_watchos_MIN_VER_FLAG}=${DARWIN_watchos_BUILTIN_MIN_VER})
     set(DARWIN_watchos_BUILTIN_ALL_POSSIBLE_ARCHS armv7 armv7k)
-    set(DARWIN_watchossim_BUILTIN_ALL_POSSIBLE_ARCHS ${X86})
+    set_simulator_arches(DARWIN_watchossim_BUILTIN_ALL_POSSIBLE_ARCHS watchos watchsimulator)
   endif()
   if(COMPILER_RT_ENABLE_TVOS)
     list(APPEND DARWIN_EMBEDDED_PLATFORMS tvos)
     set(DARWIN_tvos_MIN_VER_FLAG -mtvos-version-min)
     set(DARWIN_tvos_BUILTIN_MIN_VER 9.0)
     set(DARWIN_tvos_BUILTIN_MIN_VER_FLAG
       ${DARWIN_tvos_MIN_VER_FLAG}=${DARWIN_tvos_BUILTIN_MIN_VER})
     set(DARWIN_tvos_BUILTIN_ALL_POSSIBLE_ARCHS armv7 arm64)
-    set(DARWIN_tvossim_BUILTIN_ALL_POSSIBLE_ARCHS ${X86} ${X86_64})
+    set_simulator_arches(DARWIN_tvossim_BUILTIN_ALL_POSSIBLE_ARCHS tvos appletvsimulator)
   endif()
 
   set(BUILTIN_SUPPORTED_OS osx)
 
   # We're setting the flag manually for each target OS
   set(CMAKE_OSX_DEPLOYMENT_TARGET "")
 
   # NOTE: We deliberately avoid using `DARWIN_<os>_ARCHS` here because that is