[runtimes] Check whether -nostdinc++ and -nostdlib++ are supported

Don't blindly assume they're supported - GCC doesn't support -nostdlib++.

The llvm-project/runtimes directory is supposed to allow building the
runtimes standalone from a newly built Clang, and thus should allow
building with other compilers too.

Differential Revision: https://reviews.llvm.org/D109719