PUSHS and POPS also affect the symbol scope

Now, when POPS is executed, it restores the symbol scope of the
corresponding PUSHS. That way, the local symbols previously available
can be used again after the POPS.

This is useful in cases like this one:

```
    SECTION "Section 1", ROMX

BigFunction:

    ...

.loop:

    ...

    PUSHS

    SECTION "Section 2", ROMX

DataForBigFunction:
    DB 1, 2, 3, 4, 5

    POPS

    ld  a,BANK(DataForBigFunction)
    ld  hl,DataForBigFunction

    ...

    jr  .loop
```

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/asm/output.c

@@ -42,6 +42,7 @@ struct PatchSymbol {
 
 struct SectionStackEntry {
 	struct Section *pSection;
+	struct sSymbol *pScope; /* Section's symbol scope */
 	struct SectionStackEntry *pNext;
 };
 
@@ -64,6 +65,7 @@ void out_PushSection(void)
 		fatalerror("No memory for section stack");
 
 	pSect->pSection = pCurrentSection;
+	pSect->pScope = sym_GetCurrentSymbolScope();
 	pSect->pNext = pSectionStack;
 	pSectionStack = pSect;
 }
@@ -77,6 +79,7 @@ void out_PopSection(void)
 
 	pSect = pSectionStack;
 	out_SetCurrentSection(pSect->pSection);
+	sym_SetCurrentSymbolScope(pSect->pScope);
 	pSectionStack = pSect->pNext;
 	free(pSect);
 }