lang/gcc15: Add port

GCC, the GNU Compiler Collection, supports a number of languages.
This port installs the C, C++, and Fortran front ends as gcc15,
g++15, and gfortran15, respectively.

This is the first release from the GCC 15 series.
It largely is a copy of lang/gcc15-devel, with release-specific
modifications from lang/gcc14.

Common issues that could happen when porting code to GCC 15:
https://gcc.gnu.org/gcc-15/porting_to.html

Changes: https://gcc.gnu.org/gcc-15/changes.html

Note: files/patch-libgcc_unwind.inc will get obsolete once all supported
FreeBSD releases include commit
22e564c74eb20e14bd93fd9fdde20e38a29cfcf1. [1]

PR: 285711 [1]