[llvm] [MC] Introduce ability to add target specific AsmStreamer (PR #107415)

[via llvm-commits]

tltao wrote:

> LGTM, assuming testcases that traverse this code path will be added in the future PRs

Yeah I didn't include any test case because the behaviour in this PR is identical to the current behaviour, and I don't see a way to test anything else with the existing changes. 

https://github.com/llvm/llvm-project/pull/107415