runtime: fix crash in badsignal()

The linker can generate split stack prolog when a textflag 7 function
makes an indirect function call.  If it happens, badsignal() crashes
trying to dereference g.
Fixes #5337.

R=bradfitz, dave, adg, iant, r, minux.ma
CC=adonovan, golang-dev
https://golang.org/cl/9226043