runtime: mark {g,m,p}uintptr methods as nosplit

runtime: mark {g,m,p}uintptr methods as nosplit

These are methods that are "obviously" going to get inlined -- until you build
with -l, when they can trigger a stack split at a bad time.

Fixes #11482

Change-Id: Ia065c385978a2e7fe9f587811991d088c4d68325
Reviewed-on: https://go-review.googlesource.com/17165
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>