[MCJIT] Add a 'decodeAddend' method to RuntimeDyldMachO and teach

getBasicRelocationEntry to use this rather than 'memcpy' to get the
relocation addend. Targets with non-trivial addend encodings (E.g. AArch64) can
override decodeAddend to handle immediates with interesting encodings.

No functional change.

llvm-svn: 213435