[libspirv] Don't force fp16/fp64 OpenCL extensions

[frasercrmck]

Enabling these (or any extensions, really) by default is incorrect and prevents the r600 OpenCL builtins from being built. Each clang target reports which OpenCL extensions it does or doesn't support.

This commit also moves the flag enabling SPIR-V builtin declarations to the SPIR-V options. They are not needed for the compilation of CLC or OpenCL builtins.

There is no change to any builtins file.

[npmiller]

This looks fine, provided the extensions do get properly enabled depending on the target.

Note that r600 is a very old target, so we don't really care about it for SYCL support in general, but this is a small change and a cleanup for other targets as well so it's fine.

[frasercrmck]

This looks fine, provided the extensions do get properly enabled depending on the target.

Note that r600 is a very old target, so we don't really care about it for SYCL support in general, but this is a small change and a cleanup for other targets as well so it's fine.

The r600 target doesn't actually get built for libspirv anymore - a recent change made libspirv only build for three supported targets. The error was just that we were unconditionally enabling these OpenCL extensions for both SPIR-V and OpenCL. But since we've also enabled the 3D image writes extensions in the NVPTX clang target downstream, we can't not specify them here. It's all a bit backwards. Note that @wenju-he has an upstream PR to try and fix the 3D image writes problem: llvm/llvm-project#143331.

There's no change to any of the builtin libraries with this PR.

[wenju-he]

LGTM. Thanks @frasercrmck

[frasercrmck]

@intel/llvm-gatekeepers this is also ready to merge, thank you