[llvm-objdump] - Show aliases in -help.

Currently llvm-objdump is inconsistent.

When -help is specified it shows no aliases except two.
Aliases are shown with -help-hidden though.
GNU objdump also prints them by default.

This patch does a change to always show all aliases
when -help is given.

Differential revision: https://reviews.llvm.org/D56853

llvm-svn: 351542