runtime: copy mprof code from Go 1.7 runtime

runtime: copy mprof code from Go 1.7 runtime

    Also create a gccgo version of some of the traceback code in
    traceback_gccgo.go, replacing some code currently in C.

    This required modifying the compiler so that when compiling the runtime
    package a slice expression does not cause a local array variable to
    escape to the heap.

    Reviewed-on: https://go-review.googlesource.com/31230

From-SVN: r241189