[compiler-rt] [compiler-rt] Adds builtins support for xros. (PR #83484)

[Juergen Ributzka via llvm-commits]

ributzka wrote:

> 3. (I'll note that you don't add a min ver flag like tvOS and watchOS do, but as far as I can tell, not even Xcode clang supports `-mros-version-min=` yet, so that'll have to wait until that's added and this is fine as-is.)

I don't think there ever will be a new `-m*-version-min=`, so there is no need to add a new option for every new platform.  The new recommended approach is to use `-target` (as used by the Xcode build system) or alternatively `-arch` in combination with `-mtargetos=`.

https://github.com/llvm/llvm-project/pull/83484