runtime: disable parallel gc

Breaks on Linux/386 during parallel sync tests.

TBR=r
CC=golang-dev
https://golang.org/cl/5168044

src/pkg/runtime/proc.c

@@ -607,6 +607,7 @@ runtime·helpgc(void)
 	M *m;
 	int32 n, max;
 
+return 0;  // broken on linux/386
 	// Figure out how many CPUs to use.
 	// Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
 	max = runtime·gomaxprocs;