RFA: Breakpoint infrastructure cleanups [1/8] - define impl_breakpoint

[Andrew Cagney]

> 
> Thanks, I like it.  I'll adapt it into the next version.

Daniel,

This is like coding standards - if there is an existing convention (even 
if you personally find it a bit sucky) adopt it.  Especially when there 
there's a preference towards it by a number of developers.  It saves 
hassles trying to explain this over and over and over later.

Andrew