[compiler-rt][CMake] Remove unused -stdlib when passing -nostdinc++

[jsji]

Summary

We added -nostdinc++ to clang_rt.profile in https://reviews.llvm.org/D84205.
This will cause warnings when building with LLVM_ENABLE_LIBCXX,
and failure if with Werror on.

This patch is to fix it by removing unused -stdlib,
similar to what we have done in https://reviews.llvm.org/D42238.

[phosek]

LGTM

compiler-rt/lib/profile/CMakeLists.txt

 if(COMPILER_RT_TARGET_HAS_UNAME)
  set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_UNAME=1)
 endif()
 
 # We don't use the C++ Standard Library here, so avoid including it by mistake.
 append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ EXTRA_FLAGS)
+# Remove -stdlib= which is unused when passing -nostdinc++.
+string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
 
 # This appears to be a C-only warning banning the use of locals in aggregate
 # initializers. All other compilers accept this, though.
 # nonstandard extension used : 'identifier' : cannot be initialized using address of automatic variable
 append_list_if(COMPILER_RT_HAS_WD4221_FLAG /wd4221 EXTRA_FLAGS)
 
 if(APPLE)
   add_compiler_rt_runtime(clang_rt.profile