cmd/link/internal/ld: unconditionally use posix_fallocate on FreeBSD

cmd/link/internal/ld: unconditionally use posix_fallocate on FreeBSD

Now that Go 1.24 is the minimum bootstrap toolchain we can drop the
version dependent use of posix_fallocate on FreeBSD.

For #69315

Change-Id: Ie0c7ca67e3c21138d690e1e11a12172d52619493
Reviewed-on: https://go-review.googlesource.com/c/go/+/699735
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>