runtime: use newm for profileloop

This replaces the externalthreadhandler-based implementation of
profileloop with one that uses newm to start a new thread. This is a
step toward eliminating externalthreadhandler.

For #45530.

Change-Id: Id8e5540423fe2d2004024b649afec6998f77b092
Reviewed-on: https://go-review.googlesource.com/c/go/+/309633


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>