_bfd_mips_elf_final_link: Notify user about wrong .reginfo size

_bfd_mips_elf_final_link: Notify user about wrong .reginfo size

One of assertions in _bfd_mips_elf_final_link could be triggered by
a combination of input files and a linker script.  This happens when
either the input doesn't contain .reginfo section or when this section
is oversized.  This patch replaces the assertion with a more useful
error message.

	* elfxx-mips.c (_bfd_mips_elf_final_link): Notify user when
	.reginfo section has wrong size.