runtime: adjust traceback for new preemptive reality

Currently it crashes as follows:
fatal error: unknown pc
...
goroutine 71698 [runnable]:
runtime.racegoend()
        src/pkg/runtime/race.c:171
runtime.goexit()
        src/pkg/runtime/proc.c:1276 +0x9
created by runtime_test.testConcurrentReadsAfterGrowth
        src/pkg/runtime/map_test.go:264 +0x332

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/10674047

src/pkg/runtime/traceback_arm.c

@@ -164,7 +164,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 		waspanic = f->entry == (uintptr)runtime·sigpanic;
 
 		// Do not unwind past the bottom of the stack.
-		if(frame.pc == (uintptr)runtime·goexit || f->entry == (uintptr)runtime·mstart || f->entry == (uintptr)_rt0_go)
+		if(f->entry == (uintptr)runtime·goexit || f->entry == (uintptr)runtime·mstart || f->entry == (uintptr)_rt0_go)
 			break;
 
 		// Unwind to next frame.

src/pkg/runtime/traceback_x86.c

@@ -170,7 +170,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 			frame.fp += 2*sizeof(uintptr);
 
 		// Do not unwind past the bottom of the stack.
-		if(frame.pc == (uintptr)runtime·goexit || f->entry == (uintptr)runtime·mstart || f->entry == (uintptr)_rt0_go)
+		if(f->entry == (uintptr)runtime·goexit || f->entry == (uintptr)runtime·mstart || f->entry == (uintptr)_rt0_go)
 			break;
 
 		// Unwind to next frame.