[libclc] Have all targets build all CLC functions (#124779)
This removes all remaining SPIR-V workarounds for CLC functions, in an
effort to streamline the CLC implementation and prevent further issues
that #124614 had to fix. This commit fixes the same issue for the SPIR-V
targets.
Target-specific CLC implementations can and will exist, but for now
they're all identical and so the target-specific SOURCES files have been
removed. Target implementations now always include the 'generic' CLC
directory, meaning we can avoid unnecessary duplication of SOURCES
listings.