lang/gcc13: Add port

GCC, the GNU Compiler Collection, supports a number of languages.
This port installs the C, C++, and Fortran front ends as gcc13,
g++13, and gfortran13, respectively.

This is the first release from the GCC 13 series.
It largely is a copy of lang/gcc13-devel, with release-specific
modifications from lang/gcc12.

Common issues that could happen when porting code to GCC 13:
https://gcc.gnu.org/gcc-13/porting_to.html

Changes: https://gcc.gnu.org/gcc-13/changes.html

Reviewed by: gerald, Dan McGregor <dan.mcgregor@usask.ca>
Differential Revision: https://reviews.freebsd.org/D39841