Allow ELF64 relocation type to be more than 8 bits.

Current targets don't have more than 256 relocations so they don't hit this
limit, but ELF64 actually allows more than 8 bits for a relocation type. These
were being truncated on AArch64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171845 91177308-0d34-0410-b5e6-96231b3b80d8