[llvm-objdump] Change symbol name/PLT decoding errors to warnings

If the referenced symbol of a J[U]MP_SLOT is invalid (e.g. symbol index 0), llvm-objdump -d will bail out:

```
error: 'a': st_name (0x326600) is past the end of the string table of size 0x7
```

where 0x326600 is the st_name field of the first entry past the end of .symtab

Change it to a warning to continue dumping.
`X86/plt.test` uses a prebuilt executable, so I pick `ELF/AArch64/plt.test`
which has a YAML input and can be easily modified.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D85623