[libclc] [libclc] Fix a couple of issues preventing in-tree builds (PR #87505)

[Michał Górny via cfe-commits]

mgorny wrote:

Thanks. Unfortunately, I'm still getting a build failure:

```
[1792/1922] cd /tmp/portage/dev-libs/libclc-19.0.0.9999/work/libclc_build && /usr/lib/llvm/17/bin/llvm-spirv --spirv-max-version=1.1 -o spirv-mesa3d-.spv builtins.link.spirv-mesa3d-
FAILED: spirv-mesa3d-.spv /tmp/portage/dev-libs/libclc-19.0.0.9999/work/libclc_build/spirv-mesa3d-.spv 
cd /tmp/portage/dev-libs/libclc-19.0.0.9999/work/libclc_build && /usr/lib/llvm/17/bin/llvm-spirv --spirv-max-version=1.1 -o spirv-mesa3d-.spv builtins.link.spirv-mesa3d-
No such file or directory
```

It seems to be missing `.bc` suffix, I guess? Looking at the earlier mention, shouldn't it be `$<TARGET_FILE:${builtins_link_lib_tgt}>`?

https://github.com/llvm/llvm-project/pull/87505