[LinkerWrapper] Emit assembly files from LTO with `save-temps`

Currently in LTO mode we don't emit any `.s` files for non-NVPTX targets
during LTO. This makes it diffcult to investigate any failures in the
assembly. This patch makes the save-temps mode output an assembly file
and then assembles it separately. I decided to simply invoke `clang` for
this as it would be a lot of effort to invoke the `MCStramer` interface
directly.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D141543