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>
diff --git a/mmap/mmap_other.go b/mmap/mmap_other.go
index d1b2fe2..c2e0d5b 100644
--- a/mmap/mmap_other.go
+++ b/mmap/mmap_other.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!windows
+// +build !linux,!windows,!darwin
 
 // Package mmap provides a way to memory-map a file.
 package mmap
diff --git a/mmap/mmap_linux.go b/mmap/mmap_unix.go
similarity index 98%
rename from mmap/mmap_linux.go
rename to mmap/mmap_unix.go
index 57f87ad..4426137 100644
--- a/mmap/mmap_linux.go
+++ b/mmap/mmap_unix.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux darwin
+
 // Package mmap provides a way to memory-map a file.
 package mmap