[clang][Toolchains][Gnu] pass -g through to assembler

We've been working around this for a long time in the Linux kernel; we
bend over backwards to continue to support CC=clang (w/
-fno-integrated-as) for architectures where clang can't yet be used to
assemble the kernel's assembler sources. Supporting debug info for the
combination of CC=clang w/ GNU binutils as "GAS" has been painful.

Fix this in clang so that we can work towards dropping complexity in the
Linux kernel's build system, Kbuild, for supporting this combination of
tools.

GAS added support for -g in 2004 2.16 release via
commit 329e276daf98 ("Add support for a -g switch to GAS")

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D136309