lang/gcc15: Build jit only when bootstrap is on
Building jit without bootstrap does not work, so we add jit to the
enabled languages only if a bootstrap option has been selected.
Indeed, building GCC on FreeBSD with FreeBSD's default compiler
(clang) and without bootstraping it is not fully supported.
See e-mail
https://lists.freebsd.org/archives/dev-commits-ports-main/2025-August/160223.html
for more details.
Reported by: vishwin