Report locations for RGBLINK errors with conflicting objects (#1494)

This requires updating the object file format to record the
fstack context for sections themselves, not just for patches.

src/asm/output.cpp

@@ -85,8 +85,13 @@ static void writePatch(Patch const &patch, FILE *file) {
 }
 
 static void writeSection(Section const &sect, FILE *file) {
+	assume(sect.src->ID != (uint32_t)-1);
+
 	putString(sect.name, file);
 
+	putLong(sect.src->ID, file);
+	putLong(sect.fileLine, file);
+
 	putLong(sect.size, file);
 
 	bool isUnion = sect.modifier == SECTION_UNION;

src/asm/section.cpp

@@ -288,6 +288,8 @@ static Section *createSection(
 	sect.align = alignment;
 	sect.alignOfs = alignOffset;
 
+	out_RegisterNode(sect.src);
+
 	// It is only needed to allocate memory for ROM sections.
 	if (sect_HasData(type))
 		sect.data.resize(sectionTypeInfo[type].size);