runtime: avoid getg in preinit on Android

sigaction is called as part of library mode initializers
(_rt0_*_lib). Sigaction in turn calls getg, but on Android the TLS
offset for g has not been initialized and getg might return garbage.

Add a check for initialization before calling getg.

Fixes the golang.org/x/mobile/bind/java tests on amd64 and 386.

Fixes #31476

Change-Id: Id2c41fdc983239eca039b49a54b8853c5669d127
Reviewed-on: https://go-review.googlesource.com/c/go/+/172158Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo_sigaction.go

@@ -39,7 +39,10 @@ func sigaction(sig uint32, new, old *sigactiont) {
 
 		var ret int32
 
-		g := getg()
+		var g *g
+		if mainStarted {
+			g = getg()
+		}
 		sp := uintptr(unsafe.Pointer(&sig))
 		switch {
 		case g == nil: