[llvm] [MC][SystemZ] Introduce Target Specific HLASM Streamer for z/OS (PR #130535)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> A more fleshed out version of a previous PR https://github.com/llvm/llvm-project/pull/107415. The goal is to provide platforms an alternative to the current MCAsmStreamer which only supports the GNU Asm syntax.

Thanks for supporting z/OS HLASM without modifying the generic MC interface (the `llvm/include/llvm/MC/TargetRegistry.h` change is ok). Creating a AsmStreamer subclass from `MCStreamer` looks good to me. I have actually thought about whether we should move MCAsmStreamer to a .h file and make the class external, but that seems unnecessary for z/OS HLASM.

https://github.com/llvm/llvm-project/pull/130535