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

This is the second attempt to solve the conflict, see commits
- dc67e7d3 (first attempt);
- 9f0cd130 (revert of the first
  attempt).

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
Reviewed by:	yasu, jrm
Differential Revision:	https://reviews.freebsd.org/D37266