Sequence of operations:
- Go code does a systemstack call
- during the systemstack call, receive a signal
- signal requests a traceback of all goroutines

The orignal G is still marked as _Grunning, so the traceback code
refuses to print its stack.

Fix by allowing traceback of Gs whose caller is on the same M as G is.
G can't be modifying its stack if that is the case.

Fixes #10546

Change-Id: I2bcea48c0197fbf78ab6fa080027cd80181083ad
Reviewed-on: https://go-review.googlesource.com/9435Reviewed-by: Ian Lance Taylor <iant@golang.org>