[clang] [Clang] Make `-Xarch_` handling generic for all toolchains (PR #125421)

[David Spickett via cfe-commits]

DavidSpickett wrote:

I've pushed a fix for one of the tests on 32 bit Arm, as it failed on our bot:
https://lab.llvm.org/buildbot/#/builders/154/builds/11413/steps/5/logs/FAIL__Clang__offload-Xarch_c

Maybe your intent was specifically to have a line that uses the host architecture, if that was the case, we can figure out a `requires:` to somehow make that work. I have access to a machine to reproduce it.

The other way to fix it was to use `-Xarch_nvptx32` instead but of course that failed on a 64-bit host.

Let me know if you want to follow up on this.

https://github.com/llvm/llvm-project/pull/125421