[clang] [SFrame][Retry] Add assembler option --gsframe (PR #165806)

[via cfe-commits]

Sterling-Augustine wrote:

After some long discussion and escalation, the Project Council has agreed that SFrames can go in if I volunteer to maintain them (I do), and if I mark them as experimental. This situation is somewhat similar to the compact-relocations situation.

I have updated this PR to use `--allow-experimental-sframes`, which follows very closely after compact-relocations' `--allow-experimental-crel`.

So with that, I believe this PR is ready to be merged. If you would kindly take another look.

https://github.com/llvm/llvm-project/pull/165806