Fix up fallout from r187156.

The previous change caused the driver to translate -Wa,-L to the
-msave-temp-labels option for cc1as, but cc1as did not accept that option.
This patch follows the same approach used for similar options (-relax-all,
-noexecstack) in the previous patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191458 91177308-0d34-0410-b5e6-96231b3b80d8