Fix the build on mips, mipsel and mips64el

as a continuation of the effort that Martín Ferrari started,
see https://github.com/golang/go/issues/18031

Due to the current minimal code change to golang.org/x/sys/unix,
and to ensure the bool isBigEndian is set correctly, please run
gccgo with "-tags mips", "-tags mipsle" or "-tags mips64le"
as appropriate.