[RuntimeDyld][AArch64] Make encode/decodeAddend more typesafe by using the relocation enum type. NFCI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214204 91177308-0d34-0410-b5e6-96231b3b80d8
[RuntimeDyld][AArch64] Make encode/decodeAddend also work on big-endian hosts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214205 91177308-0d34-0410-b5e6-96231b3b80d8