runtime: rewrite panic/defer code from C to Go

runtime: rewrite panic/defer code from C to Go

    The actual stack unwind code is still in C, but the rest of the code,
    notably all the memory allocation, is now in Go.  The names are changed
    to the names used in the Go 1.7 runtime, but the code is necessarily
    somewhat different.

    The __go_makefunc_can_recover function is dropped, as the uses of it
    were removed in https://golang.org/cl/198770044.

    Reviewed-on: https://go-review.googlesource.com/33414

From-SVN: r242715