cmd/link: actually close the output file

When the output file is mmap'd, OutBuf.Close currently munmap the
file but doesn't actually close the file descriptor. This CL
makes it actually close the FD.

Change-Id: I053c5592ae95497228c50ce6a267b3b48f0af6d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/235417


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>