cmd/link: don't unmap output file at error exit

When the link exits on error it currently calls Out.Close, which
will munmap the output buffer and close the file. This may be
called in concurrent phase where other goroutines may be writing
to the output buffer. The munmap can race with the write, causing
it to write to unmapped memory and crash. This CL changes it to
just close the file without unmapping. We're exiting on error
anyway so no need to unmap.

Fixes #47816.

Change-Id: I0e89aca991bdada3d017b7d5c8efc29e46308c03
Reviewed-on: https://go-review.googlesource.com/c/go/+/363357


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>