cmd/7a: make RET an alias to ARETURN in the assembler

Note that RET still becomes ARETURN, not ARET. This means we lose
the ability to write a literal return instruction from assembly,
but that's okay, we never need to do that.

Fixes golang#28

Change-Id: I5733bbb16d98585cb7309314965750e58042a7a4

Author: 4ad

src/cmd/7a/lex.c

@@ -470,7 +470,7 @@ struct
 	"RBITW",	LTYPE2,	ARBITW,
 	"REM",	LTYPE1, AREM,
 	"REMW",	LTYPE1,	AREMW,
-	"RET",	LTYPEA,	ARET,
+	"RET",	LTYPEA,	ARETURN,
 	"REV",	LTYPE2,	AREV,
 	"REV16",	LTYPE2,	AREV16,
 	"REV16W",	LTYPE2,	AREV16W,