MIPS/BFD: Use enumeration constants for EI_ABIVERSION

MIPS/BFD: Use enumeration constants for EI_ABIVERSION

Define enumeration constants based on generated `libc-abis.h' from GNU
libc, adding a MIPS_ prefix as these are port-specific.  Use them to
replace magic numbers stored in the EI_ABIVERSION field of the ELF file
header.

	bfd/
	* elfxx-mips.c (MIPS_LIBC_ABI_DEFAULT, MIPS_LIBC_ABI_MIPS_PLT)
	(MIPS_LIBC_ABI_UNIQUE, MIPS_LIBC_ABI_MIPS_O32_FP64)
	(MIPS_LIBC_ABI_MAX): New enumeration constants.
	(_bfd_mips_post_process_headers): Use them in place of magic
	numbers.