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

[Petr Hosek via llvm-commits]

================
@@ -854,15 +854,15 @@ 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}} ${TARGET_${arch}_CFLAGS})
+      list(JOIN CMAKE_REQUIRED_FLAGS " " CMAKE_REQUIRED_FLAGS)
----------------
petrhosek wrote:

This is going to overwrite `CMAKE_REQUIRED_FLAGS` set by parent directories. Instead, we should be concatenating the flags.
```suggestion
      set(BUILTIN_CFLAGS ${BUILTIN_CFLAGS_${arch}} ${TARGET_${arch}_CFLAGS})
      list(JOIN BUILTIN_CFLAGS " " BUILTIN_CFLAGS)
      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${BUILTIN_CFLAGS}")
```

https://github.com/llvm/llvm-project/pull/133952