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

[Nico Weber via llvm-commits]

nico wrote:

1. I think you want to edit `compiler-rt/cmake/base-config-ix.cmake` to add an `option()` for this, right after `option(COMPILER_RT_ENABLE_TVOS "Enable building for tvOS - Experimental" Off)`
2. You probably want to make a similar change to yours in `compiler-rt/cmake/config-ix.cmake` as well
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.)

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