[llvm-readobj/elf] - Report "bitcode files are not supported" warning for bitcode files.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43543

Currently we report "The file was not recognized as a valid object file" for BC files.
Also, we terminate dumping.

Instead we could report a better warning and try to continue dumping other files.
This is what this patch implements.

Differential revision: https://reviews.llvm.org/D95605