Add preferred value to undef intrinsic

[lkorenc]

Currently, __remill_undef_* do not take any operands. However there are places in semantics where it would be desirable to specify a preferred value, since it could reflect the usual behaviour of cpu.
By adding extra argument to __remill_undef_* this can be communicated clearly

• information that value is supposed to be undefined is still present (and value can be safely ignored)
• it can be lowered by users who want to profit from having concrete values.

This can potentially break user code, as prototype of intrinsic will change.