libsa: smbios: Rename 64-bit entry point's guards

Rename HAS_SMBV3 to SMBIOS_64BIT_EP, as it will soon be externally
defined in some cases, and 'isv3' to 'is_64bit_ep'. These are more
accurate names for what they actually control/indicate.

No functional change.

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49281