As GCC now provides builtins for doing integer overflow checking, lets
use it when checking for overflow in xmallocarray.

Regtested on x86_64-pc-linux-gnu.

libgfortran/ChangeLog:

2019-06-14  Janne Blomqvist  <jb@gcc.gnu.org>

	* runtime/memory.c (SIZE_MAX):Remove macro definition.
	(xmallocarray): Use __builtin_mul_overflow.

From-SVN: r272295