cmd/link/internal/ld: use libc based fcntl for (*OutBuf).fallocate on darwin

Direct syscalls are no longer supported on darwin. Instead, use libc
fcntl go:linkname'd from the syscall package.

Change-Id: Ieeec64810452455faedd200f661a8b5839ca1fa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/255260


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>