[macOS] Only run Mono in 32bit mode when the GUI is launched

[DasSkelett]

Motivation

The release of macOS Catalina marked the death of 32-bit applications on Apple's platform.
This means, our GUI is no longer runnable on macOS due to Mono's WinForms implementation being 32-bit only:

#2270 hardcoded --arch=32 as argument for Mono, which now prevents users from using the CUI or TUI, if they want to.

Changes

Now there's a check for arguments in the launch script for macOS. If it's none or gui, the flag --arch=32 is used for Mono, else it's --arch=64.

Mono's man page says the following, just FYI:

--arch=32, --arch=64
(Mac  OS  X only): Selects the bitness of the Mono binary used, if available.
If the binary used is already for the selected bitness, nothing changes.
If not, the execution switches to a binary with the selected bitness suffix installed side by side
(for example, '/bin/mono --arch=64' will switch to '/bin/mono64' if '/bin/mono' is a 32-bit build).

If I understood it right that .apps behave like executables, it should enable to run ckan.app consoleui and more again.
Correct me if I'm wrong please.

Problems

This breaks commands like ckan --verbose (on macOS only, of course), which normally should run the GUI with verbose logging in the console. But I don't think a lot of people used this, and they can still add gui in the middle to make it work again, so ckan gui --verbose.

Also, I did NOT test this, since I'm lacking Apple hardware and as far as I know does Apple a good job preventing macOS to work in VMs.