[ELF][AArch64] Improve error message for unknown relocations

Like rLLD354040.

Previously, for unrecognized relocation types, in -no-pie/-pie mode, we got something like:

  foo.o: unrecognized relocation ...

In -shared mode:

  error: can't create dynamic relocation ... against symbol: yyy in readonly segment

Delete the default case from AArch64::getRelExpr and add the error there.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D66277

llvm-svn: 368983