Don't use the gcc driver for assembling.

Clang knows how to use the gnu assembler directly from doing so on linux and
hurd. The existing support worked out of the box on cygwin and mingw and I was
able to bootstrap clang with it in both systems (with pending patches for the
new mingw abi, but that is independent of the assembler).

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@195554 91177308-0d34-0410-b5e6-96231b3b80d8