Exclude more lines from test coverage (#1663)

These fall into a few categories:
- `_unreachable()`
- Verbose print messages
- Errors that should never practically occur (alloc/read/write failure,
  more than UINT32_MAX anonymous labels, etc)

src/link/assign.cpp

@@ -424,5 +424,5 @@ max_out:
 		}
 	}
 
-	unreachable_();
+	unreachable_(); // LCOV_EXCL_LINE
 }

src/link/main.cpp

@@ -397,11 +397,15 @@ int main(int argc, char *argv[]) {
 			is32kMode = true;
 			break;
 		case 'V':
+			// LCOV_EXCL_START
 			printf("rgblink %s\n", get_package_version_string());
 			exit(0);
+			// LCOV_EXCL_STOP
 		case 'v':
+			// LCOV_EXCL_START
 			beVerbose = true;
 			break;
+			// LCOV_EXCL_STOP
 		case 'w':
 			isWRAM0Mode = true;
 			break;

src/link/script.y

@@ -228,7 +228,7 @@ static uint8_t parseHexDigit(int c) {
 	} else if (c >= 'a' && c <= 'f') {
 		return c - 'a' + 10;
 	} else {
-		unreachable_();
+		unreachable_(); // LCOV_EXCL_LINE
 	}
 }
 

src/link/section.cpp

@@ -208,8 +208,10 @@ static void mergeSections(Section &target, std::unique_ptr<Section> &&other) {
 		break;
 
 	case SECTION_NORMAL:
+		// LCOV_EXCL_START
 		unreachable_();
 	}
+	// LCOV_EXCL_STOP
 
 	// Note that the order in which fragments are stored in the `nextu` list does not
 	// really matter, only that offsets were properly computed above