[llvm-readobj] - Don't abort when dumping dynamic relocations when an object has both REL and RELA.

Currently, llvm-readobj calls `report_fatal_error` when an object has
both REL and RELA dynamic relocations.

llvm-readelf is able to handle this case properly. This patch adds such a test case
and adjusts the llvm-readobj code to follow (and be consistent with its own RELR and PLTREL cases).

Differential revision: https://reviews.llvm.org/D83232