[ELF][MIPS] Create combined dynamic relocation type R_MIPS_REL32/R_MIPS_64/R_MIPS_NONE

MIPS N64 ABI packs multiple relocations into the single relocation
record. Particularly it requires to represent dynamic relative
relocation as a combination of R_MIPS_REL32 and R_MIPS_64 relocations.

llvm-svn: 268565