cmd/link: invalidate kernel cache on darwin

Apparently, the darwin kernel may cache the code signature at
mmap. When we mmap the output buffer, it doesn't have a code
signature (as we haven't generated one). Invalidate the kernel
cache after writing the file.

See https://github.com/golang/go/issues/42684#issuecomment-731704900
for more information.

Updates #38485.
Fixes #42684.

Change-Id: Iac2ef756ca1454c856944423e5040b8e17a6b420
Reviewed-on: https://go-review.googlesource.com/c/go/+/272258


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>