lang/gcc11: Fix conflict on libgccjit.h and libgccjig++.h

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

Fix conflict for GCC 11, which is GCC_DEFAULT, by installing those files
in a directory specific to GCC 11.

PR: 257060
Reported by: Ting-Wei Lan <lantw44@gmail.com>
Reviewed by: gerald, dim
Differential Revision: https://reviews.freebsd.org/D37242
See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101491