arm: cleanup build warnings

trivial stuff
lex.c: these prototypes are in a.h
asm.c: unused variables
arm-pass.txt deal w/ sieve.go rename and addition of sieve2.go

R=kaib, rsc
CC=golang-dev
https://golang.org/cl/244041

src/cmd/5a/lex.c

@@ -46,9 +46,6 @@ systemtype(int sys)
 	return sys&Plan9;
 }
 
-void*	alloc(int32);
-void*	allocn(void*, int32, int32);
-
 void
 main(int argc, char *argv[])
 {

src/cmd/5l/asm.c

@@ -195,8 +195,8 @@ vlong elfstr[NElfStr];
 void
 doelf(void)
 {
-	Sym *s, *shstrtab, *dynamic, *dynstr, *d;
-	int h, nsym, t;
+	Sym *s, *shstrtab, *dynamic, *dynstr;
+	int h, nsym;
 
 	if(!iself)
 		return;

test/arm-pass.txt

@@ -8,7 +8,8 @@ chan/perm.go
 chan/powser1.go
 chan/powser2.go
 chan/select.go
-chan/sieve.go
+chan/sieve1.go
+chan/sieve2.go
 chancap.go
 char_lit.go
 closedchan.go