mmap: use Linux support for Darwin as well.

mmap is available on Darwin using the same syscall. We can
simply build the same file to make it work on macOS as well.

Change-Id: I31bd0813efbf502287317dde8b0390c91d72aeb7
Reviewed-on: https://go-review.googlesource.com/41475
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed