lang/gcc15-devel: Allow build without bootstrap

Enabling a boostrap option was experimentally enforced on some GCC
devel ports, as building GCC on FreeBSD with FreeBSD's default compiler
(clang) is not fully supported. Stop enforcing it: many users of
production ports actually disable both bootstrap options, so it makes
sense to attempt to keep allowing no-bootstrap builds.

However, since building jit without bootstrap does not work, we add jit
to the enabled languages only if a bootstrap option has been selected [1].

See commit 5ee63cc45413954077b2b0c0546b8342585b41ba and e-mail
https://lists.freebsd.org/archives/dev-commits-ports-main/2025-August/160223.html
for more details.

Reported by: vishwin [1]