MIPS/GAS: Correct `as --help' always reporting `o32' as the default ABI
    
    Remove an issue with `as --help' always reporting `o32' as the default
    ABI regardless of what the default actually is, originally caused by
    commit cac012d6d394 ("check mips abi x linker emulation compatibility"),
    <https://sourceware.org/ml/binutils/2003-05/msg00187.html> missing an
    update here.
    
    	gas/
    	* config/tc-mips.c (md_show_usage): Correctly indicate the
    	configuration-specific default ABI.