Connect new LLVM-based libgcc_eh & libgcc_s to the build

Compiler-rt and LLVM's libunwind provide a suitable replacement for
libgcc.a, libgcc_eh.a, and libgcc_s.so.

Remove the now-unused LLVM_LIBUNWIND block from gnu/lib/libgcc.

PR: 213480 [exp-run]
Reviewed by: brooks, ed
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8189