[Hexagon] Support .reloc asm directive (#183849)
.reloc directive can be used in user assembly programs and created
internally by LLVM. In addition to Hexagon relocations, there are a few
generic names (BFD_RELOC_NONE, BFD_RELOC_8, BFD_RELOC_16,
and BFD_RELOC_32) that are mapped to Hexagon ones.
