cgen: materialise str-def operands in streq pushes

A str `def` has no name(SB) header; the streq push arms read a frame
slot that does not exist. Load the literal through cgexpr (AX=ptr,
BX=len) instead. Both stages.

selfhost/cmd/wcc/cgenexpr.ww

@@ -5808,6 +5808,14 @@ fn cgstreqpush(c: *cgen, op: *syntax.node) void = {
 			emitline("\tPUSHQ\tAX\n");
 			return;
 		};
+		if (lc == nil && deflookup(c, nm)) {
+			// A str def has no name(SB) header. Its ordinary expression
+			// load materialises the literal as AX=ptr, BX=len.
+			cgexpr(c, op);
+			emitline("\tPUSHQ\tBX\n");
+			emitline("\tPUSHQ\tAX\n");
+			return;
+		};
 		let off: i32 = 0;
 		if (lc != nil) { off = lc.off; };
 		emitline("\tMOVQ\t");