[sanitizer] Fix for CMAKE_CXX_FLAGS update

With unquoted ${CMAKE_CXX_FLAGS}, the REGEX fails when it's empty:

```CMake Error at lib/scudo/standalone/CMakeLists.txt:14 (string):
string sub-command REGEX, mode REPLACE needs at least 6 arguments total to
command.```

compiler-rt/lib/gwp_asan/CMakeLists.txt

@@ -39,7 +39,7 @@ append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC GWP_ASAN_CFLAGS)
 # append_list_if(COMPILER_RT_HAS_SANITIZER_COMMON ${SANITIZER_COMMON_CFLAGS} GWP_ASAN_CFLAGS)
 
 # Remove -stdlib= which is unused when passing -nostdinc++.
-string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 
 # Options parsing support is optional. This is an optional library that can be
 # used by an allocator to automatically parse GwpAsan options from the

compiler-rt/lib/profile/CMakeLists.txt

@@ -113,7 +113,7 @@ 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)
 # XRay uses C++ standard library headers.
-string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${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.

compiler-rt/lib/scudo/standalone/CMakeLists.txt

@@ -11,7 +11,7 @@ list(APPEND SCUDO_CFLAGS
   -nostdinc++)
 
 # Remove -stdlib= which is unused when passing -nostdinc++.
-string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 
 append_list_if(COMPILER_RT_HAS_FVISIBILITY_HIDDEN_FLAG -fvisibility=hidden SCUDO_CFLAGS)