cmd/link/internal/ld, internal/syscall/unix: use posix_fallocate on netbsd

cmd/link/internal/ld, internal/syscall/unix: use posix_fallocate on netbsd

The posix_fallocate system call is available since NetBSD 7.0, see
https://man.netbsd.org/posix_fallocate.2

Re-use the syscall wrappers already in place for freebsd. Note that
posix_fallocate on netbsd also returns the result in r1 rather than in
errno:

> If successful, posix_fallocate() returns zero. It returns an error on failure, without
> setting errno.

Source: https://man.netbsd.org/posix_fallocate.2#RETURN%20VALUES

Cq-Include-Trybots: luci.golang.try:gotip-netbsd-arm64
Change-Id: Iaa1f6a805d511645da7f1d2737235bfd42da3407
Reviewed-on: https://go-review.googlesource.com/c/go/+/480475
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
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