compiler: revamp scheme for ordering calls to import init fcns.

compiler: revamp scheme for ordering calls to import init fcns.

    Switch to a new method for determining the order in which import init
    functions are invoked: build an init fcn dependence DAG and walk the DAG
    to rewrite/adjust priorities to account for discrepancies introduced by
    "go test".

    This patch includes a change to the export data format generated
    by gccgo. Older versions of gccgo will not be able to read object files
    produced by a newer gccgo, but the new gcc will still be able to read
    old object files.

    Fixes golang/go#15738.

    Reviewed-on: https://go-review.googlesource.com/25301

From-SVN: r239708