Fix build for non windows, darwin, and linux GOOS.

All the +build OS's have syscall.MAP_ANON now so just use it.
However, netbsd was removed due to lack of syscall.SYS_MSYNC.

--HG--
branch : patch-1
Merge pull request #2 from dchapes/master

Fix build for non windows, darwin, and linux GOOS.