[libcxx][libcxxabi] CMAKE_REQUIRED_FLAGS is a string, not a list

When `libcxx` or `libcxxabi` is built with `-DLLVM_USE_SANITIZER=MemoryWithOrigins`
**and** `-DLIBCXX[ABI]_USE_COMPILER_RT=ON`, all of the `LIBCXX[ABI]_SUPPORTS_*_FLAG`
checks fail, since the value of `CMAKE_REQUIRED_FLAGS` is not set correctly.

Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=51774

Reviewed By: #libc, #libc_abi, compnerd, ldionne

Differential Revision: https://reviews.llvm.org/D109342

Author: nehaljwani

libcxx/cmake/config-ix.cmake

@@ -48,7 +48,7 @@ if (LIBCXX_SUPPORTS_NOSTDLIBXX_FLAG OR LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXX_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -rtlib=compiler-rt")
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXX_BUILTINS_LIBRARY}")
   elseif (LIBCXX_HAS_GCC_LIB)

libcxxabi/cmake/config-ix.cmake

@@ -38,7 +38,7 @@ if (LIBCXXABI_SUPPORTS_NOSTDLIBXX_FLAG OR LIBCXXABI_SUPPORTS_NODEFAULTLIBS_FLAG)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXXABI_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -rtlib=compiler-rt")
     find_compiler_rt_library(builtins LIBCXXABI_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXXABI_BUILTINS_LIBRARY}")
   else ()