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

[Artem Belevich via cfe-commits]

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@@ -1115,14 +1117,13 @@ def fno_convergent_functions : Flag<["-"], "fno-convergent-functions">,
 
 // Common offloading options
 let Group = offload_Group in {
-def offload_arch_EQ : Joined<["--"], "offload-arch=">, Flags<[NoXarchOption]>,
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Artem-B wrote:

Also, `-Xarch` has grown non-obvious usage nuances that should probably be mentioned in clang docs. We do mention them in openmp docs at the moment, but this is something that should be exposed somewhat more prominently, as this is pretty much the only mechanism we have for granular control of the offload build parameters. 

It's not uncommon to need different optimization options for different GPU variants. E.g. we want to compile with host-side debug info only, or specify different inlining/unrolling thresholds for different GPUs. All of those are effectively end-user facing options and should be documented.

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