[compiler-rt][CMake] Pass all flags to _Float16 try-compile (llvm#133952)

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_${arch}_CFLAGS` and `BUILTIN_CFLAGS_${arch}` to
`CMAKE_REQUIRED_FLAGS` before expanding CMAKE_REQUIRED_FLAGS as a
space-separated string. This passes any pre-set required flags, in addition to
all of the builtin and target flags to the Float16 detection.

(cherry picked from commit 0d3f5ec)

Author: etcwilde

compiler-rt/lib/builtins/CMakeLists.txt

@@ -849,15 +849,17 @@ 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.
+      list(APPEND CMAKE_REQUIRED_FLAGS
+        ${BUILTIN_CFLAGS_${arch}}
+        ${TARGET_${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)$")
-        string(REPLACE ";" " " _TARGET_${arch}_CFLAGS "${TARGET_${arch}_CFLAGS}")
-        check_compile_definition(__ARM_FP "${CMAKE_C_FLAGS} ${_TARGET_${arch}_CFLAGS}" COMPILER_RT_HAS_${arch}_VFP)
+        check_compile_definition(__ARM_FP "${CMAKE_C_FLAGS}" COMPILER_RT_HAS_${arch}_VFP)
         if(NOT COMPILER_RT_HAS_${arch}_VFP)
           list(REMOVE_ITEM ${arch}_SOURCES ${arm_Thumb1_VFPv2_DP_SOURCES} ${arm_Thumb1_VFPv2_SP_SOURCES} ${arm_Thumb1_SjLj_EH_SOURCES})
         else()