readelf: Handle E_MIPS_MACH_5900
    
    Fix commit e407c74b5b60 ("Support for MIPS R5900 (Sony Playstation 2)"),
    <https://sourceware.org/ml/binutils/2012-12/msg00240.html>, and add the
    handling of E_MIPS_MACH_5900, correctly showing `5900' among `Flags:' in
    the output of `-h' rather than `unknown CPU'.
    
    	binutils/
    	* readelf.c (get_machine_flags) <E_MIPS_MACH_5900>: New case.
    
    	gas/
    	* testsuite/gas/mips/elf_mach_5900.d: New test.
    	* testsuite/gas/mips/mips.exp: Run it.