runtime: add a hook to disable precise GC

This will let us ask people to rebuild the Go system without
precise GC, and then rebuild and retest their program, to see
if precise GC is causing whatever problem they are having.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8700043

src/pkg/runtime/mgc0.c

@@ -19,6 +19,7 @@ enum {
 	DebugMark = 0,  // run second pass to check mark
 	CollectStats = 0,
 	ScanStackByFrames = 0,
+	IgnorePreciseGC = 0,
 
 	// Four bits per word (see #defines below).
 	wordsPerBitmapWord = sizeof(void*)*8/4,
@@ -771,6 +772,9 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 			pc = defaultProg;
 		}
 
+		if(IgnorePreciseGC)
+			pc = defaultProg;
+
 		pc++;
 		stack_top.b = (uintptr)b;