[CMake] Don't add -lstdc++ to LDFLAGS

If the target includes a C++ source file, CMake will automatically link
the C++ standard library. This is useful when building on platforms where
libstdc++.so does not exist (e.g. CheriBSD since we only use libc++.so).

Reviewed By: smeenai
Differential Revision: https://reviews.llvm.org/D89555
49 files changed