[compiler-rt][builtins] build the macOS compiler-rt built-ins with Mac Catalyst support

This patch extends compiler-rt's cmake config to build macOS builtins with both macOS and Mac Catalyst support.
This is done by telling the compiler to emit macho files with two build version load commands.

Differential Revision: https://reviews.llvm.org/D118875

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

@@ -298,6 +298,14 @@ macro(darwin_add_builtin_library name suffix)
          -target "${LIB_ARCH}-apple-${base_os}${DARWIN_${LIBOS}_BUILTIN_MIN_VER}-simulator")
   endif()
 
+  if ("${COMPILER_RT_ENABLE_MACCATALYST}" AND
+      "${LIB_OS}" MATCHES "^osx$")
+    # Build the macOS builtins with Mac Catalyst support.
+    list(APPEND builtin_cflags
+      -target ${LIB_ARCH}-apple-macos${DARWIN_osx_BUILTIN_MIN_VER}
+      -darwin-target-variant ${LIB_ARCH}-apple-ios13.1-macabi)
+  endif()
+
   set_target_compile_flags(${libname}
     ${sysroot_flag}
     ${DARWIN_${LIB_OS}_BUILTIN_MIN_VER_FLAG}

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

@@ -3,6 +3,7 @@
 # .o files. This is particularly useful in producing larger, more complex
 # runtime libraries.
 
+include(BuiltinTests)
 include(CheckIncludeFile)
 include(CheckCXXSourceCompiles)
 include(GNUInstallDirs)
@@ -138,6 +139,12 @@ if(APPLE)
     set(OSX_SYSROOT_FLAG "")
   endif()
 
+  try_compile_only(COMPILER_RT_HAS_DARWIN_TARGET_VARIANT_FLAG
+                   FLAGS
+                   "-target" "x86_64-apple-macos10.15"
+                   "-darwin-target-variant" "x86_64-apple-ios13.1-macabi"
+                   "-Werror")
+  option(COMPILER_RT_ENABLE_MACCATALYST "Enable building for Mac Catalyst" ${COMPILER_RT_HAS_DARWIN_TARGET_VARIANT_FLAG})
   option(COMPILER_RT_ENABLE_IOS "Enable building for iOS" On)
   option(COMPILER_RT_ENABLE_WATCHOS "Enable building for watchOS - Experimental" Off)
   option(COMPILER_RT_ENABLE_TVOS "Enable building for tvOS - Experimental" Off)