This routine returns the absolute value of the two single byte numbers passed in the A and B registers.
There is a special case: 0x80 is returned as 0x7F.
A-reg contains first value
B-reg contains second value (Abs_a_b only)
A-reg contains absolute value of first value
B-reg contains absolute value of second value (Abs_a_b only)
All other registers preserved.