runtime: bypass scheduler when doing traceback for goroutine profile

When acquire a goroutine profile, we stop the world then acquire a
stack trace for each goroutine. When cgo traceback is used, the
traceback code may call the cgo traceback function using cgocall.
As the world is stopped, cgocall will be blocked at exitsyscall,
causing a deadlock. Bypass the scheduler (using asmcgocall) to fix
this.

Change-Id: Ic4e596adc3711310b6a983d73786d697ef15dd72
Reviewed-on: https://go-review.googlesource.com/c/go/+/362757


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>