Cleanup GCC compiler attributes

Added define 'unused_' for '__attribute__((unused))'. The oldest version
of GCC with online docs (GCC 2.95.3, released in March 16, 2001 [1])
already has support for this attribute, so it doesn't make sense to
check the version.

Renamed 'noreturn' to 'noreturn_' for consistency.

[1] https://gcc.gnu.org/onlinedocs/

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/link/lexer.l

@@ -179,7 +179,7 @@ void script_PrintFileStack(void)
 	fprintf(stderr, "%s(%d)", linkerscript_path, include_line[i]);
 }
 
-noreturn void script_fatalerror(const char *fmt, ...)
+noreturn_ void script_fatalerror(const char *fmt, ...)
 {
 	va_list args;
 	va_start(args, fmt);