[runtimes] adds llvm-libgcc to the list of runtimes to be sorted

llvm-libgcc is not a part of `LLVM_ALL_RUNTIMES` because llvm-libgcc is
incompatible with an explicit libunwind and compiler-rt. This meant that
it was being filtered out and not built.

Differential Revision: https://reviews.llvm.org/D128568