G M: Fix libcxx's detection of rtti disablement for g++.exe and cl.exe. When RTTI is NOT enabled, _LIBCPP_NO_RTTI is defined.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191981 91177308-0d34-0410-b5e6-96231b3b80d8