loader.efi: smbios: Favor the v3 (64-bit) entry point

Be consistent with what we are now doing with non-EFI boot (but with the
difference that EFI runs in 64-bit mode on 64-bit platforms, so there is
no restriction that the v3 entry point should be below 4GB).

While here, move out the EFI smbios detection code in a separate
sub-routine.

Reviewed by: imp, markj
MFC after: 2 weeks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49292

(cherry picked from commit 96f77576e9ea83b3a5d1a02a24da7d54c06a58a8)