Fix broken pr78138.c for avr

Fix broken pr78138.c for avr

The declaration for memcpy had the size parameter declared as
unsigned long. For avr, __SIZE_TYPE__ is unsigned int, and
this caused a builtin-declaration-mismatch warning, resulting
in a couple of FAILs.

Fixed by typedef'ing __SIZE_TYPE__ to size_t and using size_t as the
type for memcpy's third parameter.

From-SVN: r247483