runtime: find g in Windows profiler using SP

The architecture-specific interpretation of m->tls[0]
is unnecessary and fragile. Delete it.

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: I927345e52fa2f1741d4914478a29d1fb8acb0dc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/288806


Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>