runtime: fix regression on ppc64x from CL 192937

This fixes a regression introduced with CL 192937. That change
was intended to fix a problem in arm and arm64 but also added
code to change the behavior in ppc64 and ppc64le even though the
error never occurred there. The change to function sigFetchG
assumes that the register holding 'g' could be clobbered by
vdso code when in fact 'g' is in R30 and that is nonvolatile
in the 64-bit PowerPC ELF ABI so would not be clobbered in vdso code.

So if this happens somehow the path it takes is incorrect,
falling through to a call to badsignal which doesn't seem right.

This regression caused intermittent hangs on the builder dashboard
for ppc64, and can be reproduced consistently when running os/signal
TestStress on some ppc64 systems.

I mentioned this problem is issue #34391 because I thought it was
related to another problem described there.

Change-Id: I2ee3606de302bafe509d300077ce3b44b88571a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/196658Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/signal_unix.go

@@ -280,13 +280,13 @@ func sigpipe() {
 //
 //go:nosplit
 func sigFetchG(c *sigctxt) *g {
-    switch GOARCH {
-    case "arm", "arm64", "ppc64", "ppc64le":
-        if inVDSOPage(c.sigpc()) {
-            return nil
-        }
-    }
-    return getg()
+	switch GOARCH {
+	case "arm", "arm64":
+		if inVDSOPage(c.sigpc()) {
+			return nil
+		}
+	}
+	return getg()
 }
 
 // sigtrampgo is called from the signal handler function, sigtramp,