Fail-safe treat 16-bit alignment as a fixed address even in RGBLINK

RGBASM does this itself, so should not output sections with 16-bit
alignment, but an invalid object file could otherwise cause
`getPlacement` to infinitely loop.

src/link/assign.cpp

@@ -144,6 +144,8 @@ static std::optional<size_t> getPlacement(Section const &section, MemoryLocation
 				location.address = section.org;
 			} else if (section.isAlignFixed) {
 				// Move to next aligned location
+				// We have previously ensured alignment to 15 or fewer bits, so this will progress
+				assume(section.alignMask < (1 << 16) - 1);
 				// Move back to alignment boundary
 				location.address -= section.alignOfs;
 				// Ensure we're there (e.g. on first check)

src/link/section.cpp

@@ -321,6 +321,17 @@ static void doSanityChecks(Section &section) {
 			);
 		}
 	}
+
+	// An alignment of 16 is impossible to output from RGBASM, since it's treated as
+	// fixing the address, but is still satisfiable. A fixed address different from the
+	// alignment offset would not be, but we checked for that already above.
+	if (section.isAlignFixed && section.alignMask == (1 << 16) - 1) {
+		assume(!section.isAddressFixed || section.org == section.alignOfs);
+		section.isAddressFixed = true;
+		section.org = section.alignOfs;
+		section.isAlignFixed = false;
+		section.alignMask = section.alignOfs = 0;
+	}
 }
 
 void sect_DoSanityChecks() {