[ELF][MIPS] Ignore R_MIPS_JALR relocation for now

The `R_MIPS_JALR` is a relocation generated by gcc and gas. This
relocation points to the `jalr` instruction which might be optimized and
converted to the `b` instruction under some conditions.

Now we just ignore this relocation and keep instructions unchanged.

llvm-svn: 255453