runtime: simplify setPanicOnFault slightly

No need to acquire the M just to change G's paniconfault flag, and the
original C implementation of SetPanicOnFault did not. The M
acquisition logic is an artifact of golang.org/cl/131010044, which was
started before golang.org/cl/123640043 (which introduced the current
"getg" function) was submitted.

Change-Id: I6d1939008660210be46904395cf5f5bbc2c8f754
Reviewed-on: https://go-review.googlesource.com/21935
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/rdebug.go

@@ -15,9 +15,8 @@ func setMaxStack(in int) (out int) {
 
 //go:linkname setPanicOnFault runtime/debug.setPanicOnFault
 func setPanicOnFault(new bool) (old bool) {
-	mp := acquirem()
-	old = mp.curg.paniconfault
-	mp.curg.paniconfault = new
-	releasem(mp)
+	_g_ := getg()
+	old = _g_.paniconfault
+	_g_.paniconfault = new
 	return old
 }