runtime: copy cpuprof code from Go 1.7 runtime

runtime: copy cpuprof code from Go 1.7 runtime

    This replaces runtime/cpuprof.goc with go/runtime/cpuprof.go and adjusts
    the supporting code in runtime/proc.c.

    This adds another case where the compiler needs to avoid heap allocation
    in the runtime package: when evaluating a method expression into a
    closure.  Implementing this required moving the relevant code from
    do_get_backend to do_flatten, so that I could easily add a temporary
    variable.  Doing that let me get rid of Bound_method_expression::do_lower.

    Reviewed-on: https://go-review.googlesource.com/31050

From-SVN: r241163