libsa: smbios: Use 64-bit entry point if table below 4GB on non-EFI boot

On amd64, boot blocks and the non-EFI loader are 32-bit compiled as
clients of BTX, so cannot access addresses beyond 4GB. However, the
64-bit entry point may refer to a structure table below 4GB, which we
want to use if the BIOS does not provide a 32-bit entry point. The
situation is similar for powerpc64.

Consequently, always compile-in support for the 64-bit entry point, but
ensure that it is not selected on 32-bit-compiled boot loaders if the
structure table it points to grows beyond 4GB (as it is then not
accessible).

PR: 284460
Reviewed by: markj
MFC after: 2 weeks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49288