mmapfile.go uses symbols in the syscall package that are not defined
on windows and some other operating systems. Temporarily
buildtag-restrict mmapfile to darwin and linux (the platforms easiest
for me to test) to fix the build.
Change-Id: Ib056608a655b6d32170cd86deac785811e7bc0d1
Reviewed-on: https://go-review.googlesource.com/26830
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
