runtime: revert MaxGcproc from 16 to 4

The change accidentally come in with this revision:
https://code.google.com/p/go/source/detail?spec=svn345cbca96c5550f2e89bc727703301933802923c&r=14c38c23c819a17021b1808cf4a34ef3a1a17db5

R=golang-dev
CC=golang-dev
https://golang.org/cl/6195073

src/pkg/runtime/malloc.h

@@ -125,7 +125,7 @@ enum
 	// 2, 3, and 4 are all plausible maximums depending
 	// on the hardware details of the machine.  The garbage
 	// collector scales well to 4 cpus.
-	MaxGcproc = 16,
+	MaxGcproc = 4,
 };
 
 // A generic linked list of blocks.  (Typically the block is bigger than sizeof(MLink).)