lang/gcc12: Fix conflict on libgccjit.h and libgccjit++.h

GCC 11, GCC 12 and GCC 13 install libgccjit.h and libgccjit++.h in the
same directory.

Fix conflict for lang/gcc12, which is soon to become GCC_DEFAULT, by
installing those files in a directory specific to GCC 12.

See also commit 0338e04504ee269b7a95e6707f1314bc1c4239fe that fixed the
conflict in the same way for lang/gcc11.

PR: 257060