Save UNION stack when using PUSHS as well

This allows using the latter within the former

test/asm/union-pushs.asm

@@ -0,0 +1,25 @@
+
+SECTION "Test", ROM0[0]
+    dw SIZEOF("RAM")
+    dw SIZEOF("HRAM")
+
+SECTION "RAM",WRAM0
+    ds 654
+UNION
+    ds 14
+NEXTU
+    ds 897
+
+PUSHS
+
+SECTION "HRAM",HRAM
+    ds $7F
+
+POPS
+    ds 75
+NEXTU
+    ds 863
+ENDU
+
+    ds 28
+