MIPS/GAS: Correct default ABI selection for `mips64*-ps2-elf*'
    
    Correct an issue with the `mips64*-ps2-elf*' target introduced with
    commit e407c74b5b60 ("Support for MIPS R5900 (Sony Playstation 2)"),
    <https://sourceware.org/ml/binutils/2012-12/msg00240.html> and make
    the n32 ABI the default for GAS, consistently with how BFD and LD
    are configured for this target.
    
    	gas/
    	* configure.ac: Also set `mips_default_abi' to N32_ABI for
    	`mips64*-ps2-elf*'.
    	* configure: Regenerate.