Add a way to have functions with a trailing depth argument under debugging

Add a way to have functions with a trailing depth argument under debugging

In the regex engine it can be useful in debugging mode to
maintain a depth counter, but in normal mode this argument
would be unused. This allows us to define functions in embed.fnc
with a "W" flag which use _pDEPTH and _aDEPTH defines which
effectively define/pass through a U32 depth parameter to the
macro wrappers. These defines are similar to the existing
aTHX and pTHX parameters.