Fix memory region alignment checks

Turns out the checks could fail spuriously if the starting alignment is non-zero,
e.g. `align 8,$C0` in HRAM. (Fixes #2113.)
This was tripped up by a test added to exercise the linker's section placement algorithm
on a similar aligment-related issue. :D

Note that this may be caused by the code in question pre-dating alignment offsets,
and we never checked. That said, these checks only work due to the regions' starting
addresses being no more aligned than their size (..if that makes sense?), so allowing
custom memory regions (#524) could violate that assumption and make those checks incorrect.

src/asm/section.cpp

@@ -472,16 +472,15 @@ static Section *getSection(
 	if (alignment != 0) {
 		// It doesn't make sense to have both alignment and org set
 		if (org != UINT32_MAX) {
-			if ((org - alignOffset) & alignMask) {
+			if ((org & alignMask) != alignOffset) {
 				error("Section \"%s\"'s fixed address does not match its alignment", name.c_str());
 			}
 			alignment = 0; // Ignore it if it's satisfied
-		} else if (typeInfo.startAddr & alignMask) {
+		} else if ((typeInfo.startAddr & alignMask) > alignOffset) {
 			error(
 			    "Section \"%s\"'s alignment cannot be attained in %s", name.c_str(), typeInfo.name
 			);
 			alignment = 0; // Ignore it if it's unattainable
-			org = 0;
 		} else if (alignment == 16) {
 			// Treat an alignment of 16 as fixing the address.
 			alignment = 0;

src/link/section.cpp

@@ -258,7 +258,7 @@ static void doSanityChecks(Section &section) {
 	SectionTypeInfo const &typeInfo = section.typeInfo();
 
 	// Too large an alignment may not be satisfiable
-	if (section.isAlignFixed && (section.alignMask & typeInfo.startAddr)) {
+	if (section.isAlignFixed && (section.alignMask & typeInfo.startAddr) > section.alignOfs) {
 		error(
 		    "Section \"%s\" has type `%s`, which cannot be aligned to $%04x bytes",
 		    section.name.c_str(),

test/link/zero-byte-sect-constraints.asm

@@ -21,3 +21,7 @@ SECTION "free align16", ROM0, ALIGN[16,$2222] ; Should be equivalent to the abov
 
 SECTION "free align", ROM0, ALIGN[13,$1234] ; Has more than one suitable location, so cannot be trivially solved.
 	assert @ == $1234 ; Assuming that it goes into the first suitable location.
+
+
+SECTION "hram align", HRAM, ALIGN[8, $84]
+	assert @ == $FF84