[llvm] [Clang][inlineasm] Add special support for "rm" output constraints (PR #92040)

[Bill Wendling via llvm-commits]

bwendling wrote:

As mentioned in the description, this is a borderline^W hack to get this feature working for x86, which, at least in the Linux kernel, is the only architecture that uses the `"rm"` constraints.

I'm doing this partial fix to get this feature moving, but also I believe that our current support for inline asm is in need of a rewrite. However, given how intertwined inline asm support is with ISel and the register allocators, I'm not even sure it's possible to rewrite it without rewriting major portions of the back end. Even if we wanted to do this, the benefit wouldn't justify the work required.

https://github.com/llvm/llvm-project/pull/92040