runtime: setg after sigFetchG

In the normal case, sigFetchG just returns the G register. But in
the case that sigFetchG fetches the G from somewhere else, the G
register still holding an invalid value. Setg here to make sure
they match.

This is particularly useful because setGsignalStack, called by
adjustSignalStack from sigtrampgo before setg to gsignal,
accesses the G register.

Should fix #35249.

Change-Id: I64c85143cb05cdb2ecca7f9936dbd8bfec186c2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204441Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_unix.go

@@ -394,6 +394,7 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 	}
 	c := &sigctxt{info, ctx}
 	g := sigFetchG(c)
+	setg(g)
 	if g == nil {
 		if sig == _SIGPROF {
 			sigprofNonGoPC(c.sigpc())