libgo: only rebuild package if dependent .gox has changed

libgo: only rebuild package if dependent .gox has changed

    Use mvifdiff and stamp files to track whether a .gox file has changed.
    When package A depends on package B, and we rebuild package B, only
    rebuild package A if package B's .gox file changes.  This is safe
    because when package A imports package B it only reads package B's .gox
    file.  This means that changes that do not affect export information
    will not cause dependent packages to be recompiled.

    Reviewed-on: https://go-review.googlesource.com/32476

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@241742 138bc75d-0d04-0410-961f-82ee72b054a4