selfhost/cmd/{w6a,w6l}: migrate 10 amalloc sites to alloc(T{...})!

Phase 0 batch 2. Sites: w6a/parse.ww (asym, aprog, aoperand ×2),
w6l/main.ww (lnk), w6l/obj.ww (defent, armember, lobj, lrel ×2).

obj.ww's hdrsize *u8 buffer at line 344 + 7 other runtime-N byte
buffers across the three files (path bufs, n+1 cstr copies, cap
realloc) stay deferred to #8.

Verified via 991_w6a_ww + 992_w6l_ww + 995_self_rebuild +
996_dyn_ww byte-identity. make test 132/132.

selfhost/cmd/w6l/main.ww

@@ -20,8 +20,7 @@ def BASE: u64 = 4194304u64;	// 0x400000
 def CODE_VA_OFF: u64 = 4096u64;	// .text starts at base + 0x1000
 
 fn mklnk(a: *arena) *lnk = {
-	let l: *lnk = amalloc(a, 144u64): *lnk;
-	l.a = a;
+	let l: *lnk = alloc(lnk { a = a })!;
 	return l;
 };