ELF: Add a standard method for unknown relocation errors

At present each TargetRelocationHandler generates a pretty similar error
string and calls llvm_unreachable() when encountering an unknown
relocation. This is not ideal for two reasons:

1. llvm_unreachable disappears in release builds but we still want to
   know if we encountered a relocation we couldn't handle in release
   builds.

2. Duplication is bad - there is no need to have a per-architecture error
   message.

This change adds a test for AArch64 to test whether or not the error
message actually works. The other architectures have not been tested
but they compile and check-lld passes.