Re: asan: mips_gprel_reloc segfault

Similarly for the elf mips support.

	* elf32-mips.c (mips_elf_final_gp): Don't segfault on symbols
	in any of the bfd_is_const_section sections.
	* elf64-mips.c (mips_elf64_final_gp): Likewise.
	* elfn32-mips.c (mips_elf_final_gp): Likewise.