Fix BANK(@) discrepancy within LOAD blocks (#2149)

RGBASM and RGBLINK disagreed on which section to consider `@` to be in;
we have decided that the latter is correct.

The `assume` change is known to be suspicious; for more discussion, see
https://github.com/gbdev/rgbds/pull/2149#discussion_r4106902830

src/asm/output.cpp

@@ -148,7 +148,7 @@ static void initPatch(Patch &patch, uint32_t type, Expression const &expr, uint3
 
 void out_CreatePatch(uint32_t type, Expression const &expr, uint32_t ofs, uint32_t pcShift) {
 	// Add the patch to the list
-	assume(sect_GetOutputBank().has_value());
+	assume(sect_GetSymbolSection() != nullptr);
 	Patch &patch = *sect_AddOutputPatch();
 
 	initPatch(patch, type, expr, ofs);

src/asm/rpn.cpp

@@ -83,14 +83,14 @@ void Expression::makeBankSymbol(InternedStr symName) {
 	assume(rpn.empty());
 	if (Symbol const *sym = sym_FindScopedSymbol(symName); sym_IsPC(sym)) {
 		// The @ symbol is treated differently.
-		if (std::optional<uint32_t> outputBank = sect_GetOutputBank(); !outputBank) {
+		if (Section *section = sect_GetSymbolSection(); !section) {
 			error("PC has no bank outside of a section");
 			data = 1;
-		} else if (*outputBank == UINT32_MAX) {
+		} else if (section->bank == UINT32_MAX) {
 			data = "Current section's bank is not known";
 			rpn.emplace_back(RPN_BANK_SELF);
 		} else {
-			data = static_cast<int32_t>(*outputBank);
+			data = static_cast<int32_t>(section->bank);
 		}
 	} else if (sym && !sym->isLabel()) {
 		error("`BANK` argument must be a label");

src/asm/section.cpp

@@ -644,10 +644,6 @@ uint32_t sect_GetOutputOffset() {
 	return curOffset + loadOffset;
 }
 
-std::optional<uint32_t> sect_GetOutputBank() {
-	return currentSection ? std::optional<uint32_t>(currentSection->bank) : std::nullopt;
-}
-
 Patch *sect_AddOutputPatch() {
 	return currentSection ? &currentSection->patches.emplace_front() : nullptr;
 }