runtime: change some stack fields to uintptr

runtime: change some stack fields to uintptr

    Because of how gccgo implements cgo calls, the code in dropm may not
    have any write barriers.  As a step toward implementing that, change
    the gcstack, gcnextsegment, and gcnextsp fields of the g struct to
    uintptr, so that assignments to them do not require write barriers.
    The gcinitialsp field remains unsafe.Pointer, as on 32-bit systems
    that do not support split stack it points to a heap allocated space
    used for the goroutine stack.

    The test for this is runtime tests like TestCgoCallbackGC, which are
    not run today but will be run with a future gotools patch.

    Reviewed-on: https://go-review.googlesource.com/46396

From-SVN: r249561