fix(rp2040): change RSBS and SBCS according to CMP/SUBS implementation

[Turro75]

SBCS reworked to reflect latest SUBS implementation RSBS (NEGS) is actually a special case of SUBS where leftValue is 0

close #58

[urish]

This got me thinking - since all these functions share the same logic, perhaps it'll be better if way make a function (e.g. subtractUpdateFlags or something like that) that:

1. Takes leftValue and rightValue as arguments
2. Calculates the result
3. Updates the flag
4. Returns the result

Then, instructions like SUBS will use the result (e.g. store it in a register), while CMP will call this function just to update the flags, without using the result.

What do you think?

[Turro75]

that's good, I'll do it Tomorrow before the stream

[urish]

Excellent, thanks!

[Turro75]

I moved also addition instructions to a dedicated function, created a new PR