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

Like rLLD354040

Previously, for unknown 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 Hexagon::getRelExpr and add the error there. We will get consistent error message like `error: unknown relocation (1024) against symbol foo`

Reviewed By: sidneym

Differential Revision: https://reviews.llvm.org/D66275

llvm-svn: 369260