igc: Use __HAVE_ATOMIC64_LOADSTORE instead of __HAVE_ATOMIC64_OPS

to detect atomic_loadstore(9) is applicable to 64-bit integers.

Thanks riastradh@ for explanation in PR kern/58340

branches: 1.2.2;
igc(4): Add support to Intel I225 / I226 series ethernet devices

Originally written by kevlo@o for OpenBSD, and ported by knakahara@,
msaitoh@, and myself.

The driver is *EXPERIMENTAL* at the moment, as some minor error
handling paths are not fully implemented.

Hardware VLAN tagging and TSO are not supported yet.

Although, we have never observed strange behaviors at least on amd64,
aarch64{,eb}, and evbppc (IBM405), except for PR port-arm/57643.

We will send pullup request to netbsd-10, after successful snapshot
build for -current.

branches: 1.1.1;
Initial revision