[CMake][Darwin] Ignore stderr during SDKSetting.plist parsing

[steven_wu]

Summary

Don't print stderr to commandline when configuring compiler-rt for
darwin platforms. NFC.

[delcypher]

LGTM

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

if(APPLE)
   find_darwin_sdk_dir(DARWIN_tvossim_SYSROOT appletvsimulator)
   find_darwin_sdk_dir(DARWIN_tvos_SYSROOT appletvos)
 
   # Get supported architecture from SDKSettings.
   function(sdk_has_arch_support sdk_path os arch has_support)
     execute_process(COMMAND
         /usr/libexec/PlistBuddy -c "Print :SupportedTargets:${os}:Archs" ${sdk_path}/SDKSettings.plist
       OUTPUT_VARIABLE SDK_SUPPORTED_ARCHS
-      RESULT_VARIABLE PLIST_ERROR)
+      RESULT_VARIABLE PLIST_ERROR
+      ERROR_QUIET)
     if (PLIST_ERROR EQUAL 0 AND
         SDK_SUPPORTED_ARCHS MATCHES " ${arch}\n")
       message(STATUS "Found ${arch} support in ${sdk_path}/SDKSettings.plist")
       set("${has_support}" On PARENT_SCOPE)
     else()
       message(STATUS "No ${arch} support in ${sdk_path}/SDKSettings.plist")
       set("${has_support}" Off PARENT_SCOPE)
     endif()