[llvm-readelf/obj] - Fix extended section symbol indices printed in warnings...

[llvm-readelf/obj] - Fix extended section symbol indices printed in warnings for MIPS GOT/PLT entries.

Recent refactoring introduced a symbol index argument for `getFullSymbolName` method,
which is only used for reporting error messages about invalid extended symbol indexes.

There are few issues in the implementation and we don't report correct symbol indices
when dumping MIPS GOT/PLT entries currently.

This patch adds test cases and fixes the issue.

Differential revision: https://reviews.llvm.org/D88089