runtime: in doscanstackswitch, set gp->m before gogo

This is following CL 156038. doscanstackswitch uses the same
mechanism of switching goroutines as getTraceback, and so has
the same problem as described in issue golang/go#29448. This CL
applies the same fix.

Change-Id: I0da9996b0f73755f0171c7c10be6c77cb298b517
Reviewed-on: https://go-review.googlesource.com/c/156697
Reviewed-by: Ian Lance Taylor <iant@golang.org>