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>

include/asm/main.h

@@ -46,7 +46,7 @@ void opt_Parse(char *s);
  * It is also used when the assembler goes into an invalid state (for example,
  * when it fails to allocate memory).
  */
-noreturn void fatalerror(const char *fmt, ...);
+noreturn_ void fatalerror(const char *fmt, ...);
 
 /*
  * Used for errors that make it impossible to assemble correctly, but don't