[llvm-objdump] Fix the Assertion failure when providing invalid --debug-vars or --dwarf values

As seen in https://bugs.llvm.org/show_bug.cgi?id=52213 llvm-objdump
asserts if either the --debug-vars or the --dwarf options are provided
with invalid values. As suggested, this fix adds use of a default value
to these options and errors when given bad input.

Differential Revision: https://reviews.llvm.org/D112183