[Clang][Driver] Add -mcpu=help and -mtune=help to clang

Clang currently uses `-mcpu=?` and `-mtune=?`. The `?` causes errors on some
shells such as zsh since it is a special character. In order for it to work on
shells such as zsh, the option must be passed in quotes or escaped. This patch
adds `-mcpu=help` and `-mtune=help` as another alias for `--print-supported-cpus`.
In llc, `-mcpu=help` is an alias to print supported cpus.

Differential Revision: https://reviews.llvm.org/D144914