Add support for Native Client

[dcu]

I'd like to use a go library in a ChromeOS app

I got the following error when trying to compile:

$ GOOS=nacl GOARCH=amd64p32 go build
# sdk/vendor/github.com/boltdb/bolt
vendor/github.com/boltdb/bolt/db.go:97: undefined: maxMapSize
vendor/github.com/boltdb/bolt/db.go:97: invalid array bound maxMapSize

[tv42]

NaCl itself seems to support (some forms of?) mmap, but Go does not expose that functionality.