[llvm-objdump/mac] Add new function starts print mode

This updates the `--function-starts` argument to now accept 3 different
modes, `addrs` for just printing the addresses of the function starts
(previous behavior), `names` for just printing the names of the function
starts, and `both` to print them both side by side.

In general if you're debugging function starts issues it's useful to see
the symbol name alongside the address. This also mirrors Apple's
`dyldinfo -function_starts` command which prints both.

Differential Revision: https://reviews.llvm.org/D119050