add pause to nix portable code to marginally improve power consumption
and performance in the case of mp contention, on platforms supporting
pause.

Reference: /n/atom/patch/applied2013/nixpause
Date: Sun Aug 25 19:23:14 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/port/taslock.c	Sun Aug 25 19:22:19 2013
+++ /sys/src/nix/port/taslock.c	Sun Aug 25 19:22:19 2013
@@ -159,6 +159,7 @@
 				i = 0;
 				lockloop(l, pc);
 			}
+			pause();
 		}
 		if(up)
 			ainc(&up->nlocks);
@@ -202,7 +203,7 @@
 			lockstats.inglare++;
 			splx(pl);
 			while(l->key)
-				;
+				pause();
 			pl = splhi();
 			if(TAS(&l->key) == 0){
 				if(l != &waitstatslk)
@@ -320,7 +321,7 @@
 portmwait(void *value)
 {
 	while (*(void**)value == nil)
-		;
+		pause();
 }
 
 void (*mwait)(void *) = portmwait;
--- /sys/src/nix/port/sysproc.c	Sun Aug 25 19:22:21 2013
+++ /sys/src/nix/port/sysproc.c	Sun Aug 25 19:22:22 2013
@@ -848,7 +848,7 @@
 			p->rendval = PTR2UINT(va_arg(list, void*));
 
 			while(p->mach != 0)
-				;
+				pause();
 			ready(p);
 			unlock(up->rgrp);