(mips_regmask_frag): New static variable, if OBJ_ELF.
(md_begin): If OBJ_ELF, create .reginfo section and set
mips_regmask_frag to a frag.
(mips_elf_final_processing): New function, if OBJ_ELF. Set
mips_regmask_frag to register mask information.
* config/tc-mips.h (elf_tc_final_processing): New macro, defined
if OBJ_ELF.