selfhost/cmd/w6l: strip *arena cascade (γ-4)

amalloc has 0 callers post-γ-2; the *arena threaded through w6l's
mklnk/resolvelib/cstrtostr/elfglobals/dcstrtostr and the lnk.a
field are vestigial.

Drop `import mem;` from sym/main/obj/dyn/dynout, remove lnk.a
struct field, strip *arena from the five signatures, update 13
call sites. Drop a dead `let a: *arena = l.a;` in dynout. Comments
at pass.ww/obj.ww/main.ww retidied to match post-strip reality.
main.combined.ww auto-regenerated.

Verified 132/132 incl. 992_w6l_ww + 996_dyn_ww + 995_self_rebuild
byte-identity.

selfhost/cmd/w6l/sym.ww

@@ -5,8 +5,6 @@
 
 package w6l;
 
-import mem;
-
 type lsym = struct {
 	name:        str,
 	val:         u64,	// offset within combined .text (or .data when
@@ -63,7 +61,6 @@ type lso = struct {
 };
 
 type lnk = struct {
-	a:       *arena,
 	objs:    *lobj,
 	sos:     *lso,
 	syms:    *lsym,