Apply r315689 to lib/libgcc_s as well to unbreak the gcc xtoolchain build

lib/libgcc_s consumes lib/libcompiler_rt/Makefile*. The NO_WERROR.gcc in
lib/libcompiler_rt/Makefile doesn't seem to have made a difference in being
able to build this, so sprinkle NO_WERROR.gcc here as well.

MFC after: 3 days
Reported by: Jenkins (FreeBSD-head-amd64-gcc)
Tested with: amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by: Dell EMC Isilon