Fix #253: Use non-ternary varaction2 sequence for abs() builtin function

[JGRennison]

This avoids the function argument being replicated in the output and being executed at run-time three times.

[glx22]

I did a quick test by changing https://github.com/OpenTTD/nml/blob/master/regression/019_switch.nml#L10 from return 0 to return abs(current_year)

Before:

// Name: @return_action_0
4 * 107 02 0A FF 89
24 20 \dxFFFFFFFF
\2cmp 1A 20 \dx00000000
\2< 1A 20 \dx00000001
\2^ 1A 20 \dx00000001
\2sto 1A 20 \dx00000080 	// guard
\2^ 1A 20 \dx00000001
\2sto 1A 20 \dx00000081 	// !guard
\2r 7D 81 20 \dxFFFFFFFF
\2* 24 20 \dxFFFFFFFF
\2sto 1A 20 \dx00000082
\2r 1A 20 \dx00000000
\2- 24 20 \dxFFFFFFFF
\2* 7D 80 20 \dxFFFFFFFF
\2+ 7D 82 00 \dxFFFFFFFF
\b0
\wx8000 // Return computed value

After:

// Name: @return_action_0
4 * 42 02 0A FF 89
24 20 \dxFFFFFFFF
\2sto 1A 20 \dx00000080 	// input
\2cmp 1A 20 \dx00000000
\2- 1A 20 \dx00000001
\2* 7D 80 00 \dxFFFFFFFF
\b0
\wx8000 // Return computed value

And this was a very simple expression, I can see how it's even better for size and execution with a complex expression.