[compiler-rt] Adds builtins support for xros.

[rohit-rao]

Adds support for xros when compiling builtins. This is disabled by default and controlled with COMPILER_RT_ENABLE_XROS, similar to watchOS/tvOS.

[rohit-rao]

@delcypher Would like to have support for xros, please take a look when you have a minute. Thanks!

[nico]

@jroelofs fyi

[nico]

@yln @kubamracek fyi too :)

[nico]

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.)

[rohit-rao]

Updated to add the option and fix supported archs. I'll add sanitizer support in a separate CL, if that's ok.

Agreed, as far as I can tell, there is no min version flag for xros, so I left it out for now.

[ributzka]

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=.

[ributzka]

See https://reviews.llvm.org/D106316

[nico]

This has been sitting for a while and got some comments that got addressed, so I'll merge this. Shout if you want follow-up work to happen :)