Improve cli argument parsing

[Urhengulas]

Currently we have various limitations of how to specify cli arguments. An out-of-the-box solution like clap¹ would be helpful.
One limitation example is that it's only possible to pass the linker script as -Tmemory.x, but not as -T memory.x.

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¹ - clap in particular isn't currently easily implementable (see #29 for one approach), but would be with one of clap-rs/clap#1880, clap-rs/clap#1404.

[Urhengulas]

Also this feature should help removing this hack:

flip-link/src/main.rs

Lines 108 to 109 in 3e43bab

	// HACK `-L` needs to go after `-flavor gnu`; position is currently hardcoded
	c2.args(&args[..2])