llvmPackages/libc++: Fix build against glibc 2.26

This is very similar to what we had in bb0b0822

.

The xlocale.h header is no longer existing in glibc version 2.26, so we
need to avoid including it.

I've tested building against all of the libcxx attributes of LLVM 3.5,
3.7, 3.8, 3.9, 4 and 5.

All of them succeeded except version 3.5, which failed because of an
unrelated issue (build of libc++abi has failed, one of its
dependencies), so I only verified whether the patch applies cleanly.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @vcunat