Use __FreeBSD_version to determine if gets() has been removed.

GCC compilers set FreeBSD statically to a build-time determined
targeted version (which in ports always matches the build host's
version). This means that when building any version (12 or 13, etc.)
of riscv or some other architecture via GCC on a 12.x host,
FreeBSD will always be set to 12. As a result, FreeBSD cannot
be used to reliably detect the target FreeBSD version being built.
Instead, __FreeBSD_version from either <sys/param.h> (in the kernel)
or <osreldate.h> (in userland) should be used.

This changes the gets() test in libc++ to use __FreeBSD_version from
<osreldate.h>.

Reported by: jenkins (riscv64 and amd64-gcc)
Reviewed by: dim, imp
Differential Revision: https://reviews.freebsd.org/D22034