runtime: in getTraceback, set gp->m before gogo

runtime: in getTraceback, set gp->m before gogo

    Currently, when collecting a traceback for another goroutine,
    getTraceback calls gogo(gp) switching to gp, which will resume in
    mcall, which will call gtraceback, which will set up gp->m. There
    is a gap between setting the current running g to gp and setting
    gp->m. If a profiling signal arrives in between, sigtramp will
    see a non-nil gp with a nil m, and will seg fault. Fix this by
    setting up gp->m first.

    Fixes golang/go#29448.

    Reviewed-on: https://go-review.googlesource.com/c/156038

From-SVN: r267658