[MC][Mips] Support .reloc *, BFD_RELOC_{NONE,16,32,64}, *

... to match most other common architectures which already support BFD_RELOC_*.
BFD_RELOC_NONE provides a generic way indicating a dependency between two
sections and is useful for some instrumentations which encode symbol index
information (e.g. `.cg_profile`).