Add a new sysctl in order to diffrentiate UEFI architectures

With the new 32-bit UEFI loader, it's convenient to have a sysctl to
figure out how we booted. Can be accessed at machdep.efi_arch

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1098

(cherry picked from commit b538d4911004ca541507166b8ec9689d2e87d1aa)
(cherry picked from commit 0e8890a425bc778d323566c881e26bad7c54baf2)

Approved by: so
Security: FreeBSD-EN-25:12.efi