runtime: handle nil gp in cpuprof

This can happen on Windows when recording profile samples for system threads.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I5a7ba32b1900a69f3b7acada9cb6cf8396d8a03f
Reviewed-on: https://go-review.googlesource.com/c/go/+/288797


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>