runtime: save correct pid for new m's on plan9/amd64

The pid field in the Tos structure is a 32-bit value.
Loading a 64-bit word also brings in the next field
which is used for the profiling clock.

LGTM=0intro, aram
R=rsc, 0intro, aram
CC=golang-codereviews, mischief
https://golang.org/cl/139560044

src/runtime/sys_plan9_amd64.s

@@ -149,7 +149,7 @@ TEXT runtime·tstart_plan9(SB),NOSPLIT,$0
 
 	// Initialize procid from TOS struct.
 	MOVQ	_tos(SB), AX
-	MOVQ	64(AX), AX
+	MOVL	64(AX), AX
 	MOVQ	AX, m_procid(CX)	// save pid as m->procid
 
 	// Finally, initialize g.