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

This is the second attempt to solve the conflict, see commits

• dc67e7d3009de2b78d96136c8ad09cf186e553df (first attempt);
• 9f0cd13014d0962aedfa016c86a5af07ca3ba814 (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