#1026 Deal with side-channels in mcs51 port

[Philipp Klaus Krause]

Now that work on [feature-requests:#1006] has started, we should also look into the z80 (and related) -specific aspects. We then need fixes or documentation. A first view shows:

• Side channels are created in trunk codegen for negation. These will be fixed in the nosidechannels branch today.
• There are side-channels in codegen for multiplication, division, modulo, addition, subtraction comparison, shifts (right operand leaked), reads-from signed bit-fields, signed casts
• There are no side-channels introduced in peephole optimizer rules
• There are side-channels in support routines.