[libclc] [libclc] Move CMake for prepare_builtins to a subdirectory (PR #148815)

[Michał Górny via cfe-commits]

mgorny wrote:

This broke the standalone builds on Gentoo Linux amd64:

```
[1566/1676] cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
FAILED: clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc 
cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
/bin/sh: line 1: -o: command not found
```

https://github.com/llvm/llvm-project/pull/148815