Build OmpSs-2 LLVM with commit version information

Allows users to see which commit (or git tag) was used in clang.
Examples for the release and git versions:

% clang --version
clang version 18.0.0 (18.0.0-ompss-2)

% clang --version
clang version 18.0.0 (0a6d6c6)

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>