add clrex instruction

Reference: /n/atom/patch/applied/5aclrex
Date: Fri Feb  6 06:05:03 CET 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/5a/lex.c	Fri Feb  6 06:04:54 2015
+++ /sys/src/cmd/5a/lex.c	Fri Feb  6 06:04:55 2015
@@ -380,6 +380,7 @@
 
 	"RET",		LTYPEA, ARET,
 	"RFE",		LTYPEA, ARFE,
+	"CLREX",	LTYPEA, ACLREX,
 
 	"TEXT",		LTYPEB, ATEXT,
 	"GLOBL",	LTYPEB, AGLOBL,