Parse MIPS relocations to unbreak kldxref on MIPS.

Parse the R_MIPS_32 and R_MIPS_64 relocations. Both Elf_Rel and
Elf_Rela relocations are handled since O32 MIPS uses Elf_Rel while N64
uses Elf_Rela. Note that R_MIPS_32 is only handled for 32-bit mips
and R_MIPS_64 for 64-bit. N32 is untested.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D19870