Fix bogus pr78886.c failure for avr

Fix bogus pr78886.c failure for avr

The test fails for avr because malloc has been declared to take
an unsigned long, whereas size_t for avr is only unsigned int.

Fixed by typedef'ing __SIZE_TYPE__ to size_t and using it in the
malloc function declaration.

gcc/testsuite/

2017-05-17  Senthil Kumar Selvaraj  <senthil_kumar.selvaraj@atmel.com>

	* gcc.dg/tree-ssa/pr78886.c: Use __SIZE_TYPE__ instead of
	unsigned long.

From-SVN: r248138