selfhost: mirror defer; e2e tests for LIFO ordering

C cgen has carried defer for a while (defers[] global + reverse
walk on every return). Selfhost cgen now mirrors:

- cgen struct: deferbuf (**node, LIFO stack) + defertop counter.
- cgstmt N_DEFER: push n.lhs.
- cgreturn: rundefers() at entry — same as the C cgen pattern.
- cgfn fall-through return: rundefers() before zero-AX+RET.

DEFER_MAX = 16 matches C cgen.

Two new e2e rows: defer with an explicit `return acc;` (321 mod 256
= 65), and defer firing on an implicit void-fn fall-through (87).
Both rows verified via the wwstage cgen too.

Defer's semantics: queued exprs fire LIFO before the return expr
is evaluated, so a return that reads memory mutated by a deferred
call sees the post-defer state. Matches C cgen and Hare.

selfhost/cmd/wcc/cgendecl.ww

@@ -258,6 +258,9 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 	if (fn_.body != nil) { cgstmt(c, fn_.body); };
 
 	if (c.lastwasreturn == 0) {
+		// Run any registered defers in LIFO order before the
+		// implicit return.
+		rundefers(c);
 		// Zero AX before the fall-through return — matches C cgen,
 		// which always emits this so void-returning fns don't leak
 		// a stale callee value to their caller.