[compiler-rt][CMake] Pass all flags to try-compile

The try-compile mechanism requires that `CMAKE_REQUIRED_FLAGS` is a
space-separated string instead of a list of flags. The original code
expanded BUILTIN_FLAGS into CMAKE_REQUIRED_FLAGS as a space-separated
string and then would overwrite CMAKE_REQUIRED_FLAGS with
TARGET_${arch}_CFLAGS prepended to the unexpanded
BUILTIN_CFLAGS_${arch}. This resulted in the first two arguments being
passed into the try-compile invocation, but dropping the other arguments
listed in BUILTIN_CFLAGS_${arch}.

This patch appends ${TARGET_${arch}_CFLAGS} to the list, and then joins
the elements with spaces, resulting in all arguments passed via
CMAKE_REQUIRED_FLAGS being space-separated and being used in the
try-compile when detecting Float16.

Author: etcwilde

compiler-rt/lib/builtins/CMakeLists.txt

@@ -854,10 +854,12 @@ else ()
       cmake_push_check_state()
       # TODO: we should probably make most of the checks in builtin-config depend on the target flags.
       set(BUILTIN_CFLAGS_${arch} ${BUILTIN_CFLAGS})
-      # CMAKE_REQUIRED_FLAGS must be a space separated string but unlike TARGET_${arch}_CFLAGS,
-      # BUILTIN_CFLAGS_${arch} is a CMake list, so we have to join it to create a valid command line.
-      list(JOIN BUILTIN_CFLAGS " " CMAKE_REQUIRED_FLAGS)
-      set(CMAKE_REQUIRED_FLAGS "${TARGET_${arch}_CFLAGS} ${BUILTIN_CFLAGS_${arch}}")
+      # CMAKE_REQUIRED_FLAGS must be a space separated string
+      # Join BUILTIN_CFLAGS_${arch} and TARGET_${arch}_CFLAGS as a
+      # space-separated string.
+      set(CMAKE_REQUIRED_FLAGS ${BUILTIN_CFLAGS_${arch}})
+      list(APPEND CMAKE_REQUIRED_FLAGS ${TARGET_${arch}_CFLAGS})
+      list(JOIN CMAKE_REQUIRED_FLAGS " " CMAKE_REQUIRED_FLAGS)
       message(STATUS "Performing additional configure checks with target flags: ${CMAKE_REQUIRED_FLAGS}")
       # For ARM archs, exclude any VFP builtins if VFP is not supported
       if (${arch} MATCHES "^(arm|armhf|armv7|armv7s|armv7k|armv7m|armv7em|armv8m.main|armv8.1m.main)$")