[builtins] Build for arm64_32 for watchOS (Darwin)

[drodriguez]

Summary

Trying to build the builtins code fails because arm64_32_SOURCES is
missing. Setting it to the same list used for aarch64_SOURCES solves
that problem and allow the builtins to compile for that architecture.

Additionally, arm64_32 is added as a possible architecture for watchos
platforms.

[compnerd]

Thanks!

[delcypher]

@drodriguez In future could you add me as a reviewer to Apple CMake changes as well?

[drodriguez]

@delcypher: I will. Is there any concerns about the code as landed?

[delcypher]

The code in compiler-rt/cmake/builtin-config-ix.cmake is reasonable. Although we (Apple) should probably upstream our change because it's slightly different. I'm not sure about the change in compiler-rt/lib/builtins/CMakeLists.txt we don't have the change you made. I'll put a patch up to upstream our version and we'll see how it goes.

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

if(COMPILER_RT_ENABLE_IOS)
     set(DARWIN_iossim_BUILTIN_ALL_POSSIBLE_ARCHS ${X86} ${X86_64})
   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_watchos_BUILTIN_ALL_POSSIBLE_ARCHS armv7 armv7k arm64_32)
     set(DARWIN_watchossim_BUILTIN_ALL_POSSIBLE_ARCHS ${X86})
   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})

compiler-rt/lib/builtins/CMakeLists.txt

 endif()
 
 set(armhf_SOURCES ${arm_SOURCES})
 set(armv7_SOURCES ${arm_SOURCES})
 set(armv7s_SOURCES ${arm_SOURCES})
 set(armv7k_SOURCES ${arm_SOURCES})
 set(arm64_SOURCES ${aarch64_SOURCES})
 set(arm64e_SOURCES ${aarch64_SOURCES})
+set(arm64_32_SOURCES ${aarch64_SOURCES})
 
 # macho_embedded archs
 set(armv6m_SOURCES ${thumb1_SOURCES})
 set(armv7m_SOURCES ${arm_SOURCES})
 set(armv7em_SOURCES ${arm_SOURCES})
 
 # hexagon arch
 set(hexagon_SOURCES