AArch64: make RuntimeDyld relocations idempotent

AArch64 ELF uses .rela relocations so there's no need to actually make
use of the bits we're setting in the destination  However, we should
make sure all bits are cleared properly since multiple runs of
resolveRelocations are possible and these could combine to produce
invalid results if stale versions remain in the code.

llvm-svn: 182214