[Clang] Make offloading flags accept '-' and '--'

Currently all of the flags beginning with `--offload` such as
`--offload-arch` or `--offload-device-only` require the double-dash
form. However, if a user uses a single dash it will instead name a file
'ffload-arch' for example. This has been the cause of a lot of user
confusion. This patch changes these options to also accept a single
dash. This is similar to many other driver arguments beginning with
`-o`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D135076