mips clone: don't free stack space used to copy arg

the mips abi reserves stack space equal to the size of the in-register
args for the callee to save the args, if desired. this would cause the
beginning of the thread structure to be clobbered...

src/thread/mips/clone.s

@@ -25,5 +25,4 @@ __clone:
 1:	lw $25, 0($sp)
 	lw $4, 4($sp)
 	jr $25
-	addu $sp, $sp, 16
 	nop