[Gnu toolchain] Look at standard GCC paths for libstdcxx by default

Linux' current addLibCxxIncludePaths and addLibStdCxxIncludePaths
are actually almost non-Linux-specific at all, and can be reused
almost as such for all gcc toolchains. Only keep
Android/Freescale/Cray hacks in Linux's version.

Patch by sthibaul (Samuel Thibault)

Differential Revision: https://reviews.llvm.org/D69758