[libclc] Move sources collection into libclc_add_builtin_library, merge opencl library creation into libclc_add_library (#197654)
Merging opencl library creation into libclc_add_library undos part of
change in 14bf8e7.
This simplifies creating different libraries by invoking
libclc_add_library multiple times, without the need of splitting
intermediate library creation and its finalization.
add_subdirectory(clc) and add_subdirectory(opencl) only updates sources
list of existing library target, so it is safe to move inside
libclc_add_builtin_library.