target/10338: [3.3 regression?] [Cygwin -> tic4x | avr] cross target compilation error

[DJ Delorie]

> So I don't think it's a regression.  It seems to be present on all of the 
> 3.2.x branches, so I'm guessing it's a Cygwin problem.  I worked around it 
> similarly by just not using __memcpy(), and instead memcpy() in the #else 
> branch.

It looks like it's always been a bug, but until now there hasn't been
an OS that defined mempcpy (not memcpy) and not also __mempcpy, so we
just didn't trip over it.