[libc++] Disable -Wattributes in GCC

Currently GCC produces lots of warnings. Most of them are `-Wattributes`, but these warnings are completly ignored by everybody. So let's disable -Wattributes and make the output cleaner.

Reviewed By: ldionne, #libc

Spies: libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D119140