Ensure that object file section IDs are correct

[Rangi42]

I can't make a test case to demonstrate this because it doesn't currently occur, since RGBASM merges section fragments (and unions) at assembly time when they're in the same object:

	// Check if another section exists with the same name; merge if yes, otherwise create one

	Section *sect = sect_FindSectionByName(name);

	if (sect) {
		mergeSections(*sect, type, org, bank, alignment, alignOffset, mod);
	} else {
		sect = createSection(name, type, org, bank, alignment, alignOffset, mod);
	}

However, this is a theoretically correct fix which would avoid bugs if that ever changes.

[Rangi42]

This change/fix was motivated by attempting to rebase #716 to the current master. Back when it was coded, section IDs were derived by searching through a linked list for a specific Section * pointer, which naturally distinguished between multiple fragments with the same name. The current master instead finds whichever section ID corresponds to the first fragment/union with that name. "Inline fragments"/"fragment literals" are one feature which expose this difference, but there could theoretically be others.