lang/gcc16: Add port

GCC, the GNU Compiler Collection, supports a number of languages.
This port installs the C, C++, and Fortran front ends as gcc16,
g++16, and gfortran16, respectively.

This is the first release from the GCC 16 series.
It largely is a copy of lang/gcc16-devel, with release-specific
modifications from lang/gcc15.

Common issues that could happen when porting code to GCC 16:
https://gcc.gnu.org/gcc-16/porting_to.html

Changes: https://gcc.gnu.org/gcc-16/changes.html