selfhost/cmd/wcc: migrate 2 cgenutil amalloc sites to alloc(T{...})!

Phase 0 batch 3a. structinfo registration + fieldinfo per-field in
registerstruct (cgenutil.ww). Both relied on amalloc-zero for fields=nil
and totsize=0 (structinfo) and finext=nil (fieldinfo); MAP_ANON-zero
covers the same slots.

check.ww:842 (arenau64tos 24B scratch) deferred to #11.
cgenutil.ww:108 (mkvarargname runtime-N) deferred to #8.

Verified 132/132 + 995_self_rebuild byte-identity.

selfhost/cmd/w6c/main.combined.ww

@@ -11392,11 +11392,10 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 };
 
 fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
-	let si: *structinfo = amalloc(c.a, 80u64): *structinfo;
-	si.sname = name;
-	si.smod = srcmod;
-	si.fields = nil;
-	si.totsize = 0;
+	let si: *structinfo = alloc(structinfo{
+		sname = name,
+		smod = srcmod,
+	})!;
 	let head: *fieldinfo = nil;
 	let tail: *fieldinfo = nil;
 	let off: i32 = 0;
@@ -11414,11 +11413,12 @@ fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
 			if ((off & (aln - 1)) != 0) {
 				off = (off + aln - 1) & ~(aln - 1);
 			};
-			let fi: *fieldinfo = amalloc(c.a, 48u64): *fieldinfo;
-			fi.fname = f.str;
-			fi.foff = off;
-			fi.fsz  = sz;
-			fi.tnode = f.lhs;
+			let fi: *fieldinfo = alloc(fieldinfo{
+				fname = f.str,
+				foff  = off,
+				fsz   = sz,
+				tnode = f.lhs,
+			})!;
 			if (head == nil) { head = fi; tail = fi; }
 			else { tail.finext = fi; tail = fi; };
 			off += sz;

selfhost/cmd/wcc/cgenutil.ww

@@ -2181,11 +2181,10 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 };
 
 fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
-	let si: *structinfo = amalloc(c.a, 80u64): *structinfo;
-	si.sname = name;
-	si.smod = srcmod;
-	si.fields = nil;
-	si.totsize = 0;
+	let si: *structinfo = alloc(structinfo{
+		sname = name,
+		smod = srcmod,
+	})!;
 	let head: *fieldinfo = nil;
 	let tail: *fieldinfo = nil;
 	let off: i32 = 0;
@@ -2203,11 +2202,12 @@ fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
 			if ((off & (aln - 1)) != 0) {
 				off = (off + aln - 1) & ~(aln - 1);
 			};
-			let fi: *fieldinfo = amalloc(c.a, 48u64): *fieldinfo;
-			fi.fname = f.str;
-			fi.foff = off;
-			fi.fsz  = sz;
-			fi.tnode = f.lhs;
+			let fi: *fieldinfo = alloc(fieldinfo{
+				fname = f.str,
+				foff  = off,
+				fsz   = sz,
+				tnode = f.lhs,
+			})!;
 			if (head == nil) { head = fi; tail = fi; }
 			else { tail.finext = fi; tail = fi; };
 			off += sz;

selfhost/cmd/wwdump/main.combined.ww

@@ -11392,11 +11392,10 @@ fn fieldsize(c: *cgen, tnode: *node) i32 = {
 };
 
 fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
-	let si: *structinfo = amalloc(c.a, 80u64): *structinfo;
-	si.sname = name;
-	si.smod = srcmod;
-	si.fields = nil;
-	si.totsize = 0;
+	let si: *structinfo = alloc(structinfo{
+		sname = name,
+		smod = srcmod,
+	})!;
 	let head: *fieldinfo = nil;
 	let tail: *fieldinfo = nil;
 	let off: i32 = 0;
@@ -11414,11 +11413,12 @@ fn registerstruct(c: *cgen, name: str, srcmod: str, tstruct: *node) void = {
 			if ((off & (aln - 1)) != 0) {
 				off = (off + aln - 1) & ~(aln - 1);
 			};
-			let fi: *fieldinfo = amalloc(c.a, 48u64): *fieldinfo;
-			fi.fname = f.str;
-			fi.foff = off;
-			fi.fsz  = sz;
-			fi.tnode = f.lhs;
+			let fi: *fieldinfo = alloc(fieldinfo{
+				fname = f.str,
+				foff  = off,
+				fsz   = sz,
+				tnode = f.lhs,
+			})!;
 			if (head == nil) { head = fi; tail = fi; }
 			else { tail.finext = fi; tail = fi; };
 			off += sz;