runtime: simplify some pointer conversions

Use efaceOf to safely convert from *interface{} to *_eface, and to
make it clearer what the pointer arithmetic is computing.

Incidentally, remove a spurious unsafe.Pointer->*uint8->unsafe.Pointer
round trip conversion in newproc.

No behavior change.

Change-Id: I2ad9d791d35d8bd008ef43b03dad1589713c5fd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/190457


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>