libgo: build roots index to speed up bulkBarrierPreWrite

libgo: build roots index to speed up bulkBarrierPreWrite

    To reduce the amount of time spent in write barrier processing
    (specifically runtime.bulkBarrierPreWrite), add support for building a
    'GC roots index', basically a sorted list of all roots, so as to
    allow more efficient lookups of gcdata structures for globals. The
    previous implementation worked on the raw (unsorted) roots list
    itself, which did not scale well.

    Reviewed-on: https://go-review.googlesource.com/132595

From-SVN: r264276