[libclc] Clean up directory search procedure (#127783)

During a recent change, the build system accidentally dropped the
(theoretical) support for the CLC builtins library to build
target-specific builtins from the 'amdgpu' directory, due to a change in
variable names. This functionality wasn't being used but was spotted
during another code review.

This commit takes the opportunity to clean up and better document the
code that manages the list of directories to search for builtin
implementations.

While fixing this, some references to now-removed SOURCES files were
discovered which have been cleaned up.

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