[dev.power64] runtime: fix fastrand1 on power64x

fastrand1 depends on testing the high bit of its uint32 state.
For efficiency, all of the architectures implement this as a
sign bit test.  However, on power64, fastrand1 was using a
64-bit sign test on the zero-extended 32-bit state.  This
always failed, causing fastrand1 to have very short periods
and often decay to 0 and get stuck.

Fix this by using a 32-bit signed compare instead of a 64-bit
compare.  This fixes various tests for the randomization of
select of map iteration.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/166990043

src/runtime/asm_power64x.s

@@ -965,7 +965,7 @@ TEXT runtime·fastrand1(SB), NOSPLIT, $0-4
 	MOVD	g_m(g), R4
 	MOVWZ	m_fastrand(R4), R3
 	ADD	R3, R3
-	CMP	R3, $0
+	CMPW	R3, $0
 	BGE	2(PC)
 	XOR	$0x88888eef, R3
 	MOVW	R3, m_fastrand(R4)