[compiler-rt] Quote multiple warning flags in check_cxx_compiler_flag invocation

Without this all checks fail because CMake passes the flags
like this:
`... -nodefaultlibs   -D-Warray-bounds -Werror -std=c++17 ...`
Note the `-D` before the `-W`.

Reviewed By: ahatanak

Differential Revision: https://reviews.llvm.org/D146920