fix child stack alignment on mips clone

unlike other archs, the mips version of clone was not doing anything
to align the stack pointer. this seems to have been the cause for some
SIGBUS crashes that were observed in posix_spawn.

src/thread/mips/clone.s

@@ -3,6 +3,7 @@
 .type   __clone,@function
 __clone:
 	# Save function pointer and argument pointer on new thread stack
+	and $5, $5, -8
 	subu $5, $5, 16
 	sw $4, 0($5)
 	sw $7, 4($5)