ARM: use pristine object file while processing relocations

Previously we would read-modify-write the target bits when processing
relocations for the MCJIT. This had the problem that when relocations
were processed multiple times for the same object file (as they can
be), the result is not idempotent and the values became corrupted.

The solution to this is to take any bits used in the destination from
the pristine object file as LLVM emitted it.

This should fix PR16013 and remote MCJIT on ARM ELF targets.