runtime: fix float64 hash on 32-bit machine

Multiplying by the low 32 bits was a bad idea
no matter what, but it was a particularly unfortunate
choice because those bits are 0 for small integer values.

Fixes #2883.

R=ken2
CC=golang-dev
https://golang.org/cl/5634047

src/pkg/runtime/alg.c

@@ -271,7 +271,7 @@ runtime·f64hash(uintptr *h, uintptr s, void *a)
 	else {
 		u = *(uint64*)a;
 		if(sizeof(uintptr) == 4)
-			hash = ((uint32)(u>>32) ^ 2860486313) * (uint32)u;
+			hash = ((uint32)(u>>32) * 3267000013UL) ^ (uint32)u;
 		else
 			hash = u;
 	}