MIPS/GAS: Correct default ABI selection for `mips64*-ps2-elf*'

Correct an issue with the `mips64*-ps2-elf*' target introduced with
commit e407c74b ("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.