libsa: smbios: Favor the v3 (64-bit) entry point on non-EFI boot

When both the 32-bit and 64-bit entry points are present, the SMBIOS
specification says that the 64-bit entry point always has at least all
the structures the 32-bit entry point refers. In other words, the
32-bit entry point is provided for compatibility, so we assume the
64-bit one has more chances to be filled with adequate values.

Doing this also increases consistency with the kernel's smbios(4)
driver.

Reviewed by: imp, markj
MFC after: 2 weeks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49287