[MonoPosixHelper] Allow MmapFlags.MAP_ANON use on OS X.

[MonoPosixHelper] Allow MmapFlags.MAP_ANON use on OS X.

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=3419

The problem is that OS X defines MAP_ANON but not MAP_ANONYMOUS, AND
MAP_ANON and MAP_ANONYMOUS have the same value. Furthermore,
create-native-map doesn't have any special support for constants with
the same value, so it emits the validation independently.

The result is that the following fails on OS X:

	$ csharp -r:Mono.Posix.dll
	csharp> int r;
	csharp> using Mono.Unix.Native;
	csharp> NativeConvert.TryFromMmapFlags(MmapFlags.MAP_ANONYMOUS, out r);
	false
	csharp> r;

The (hackish) fix is to provide MAP_ANONYMOUS on OS X, thus allowing
the existing validation logic to work as expected.