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
