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 b538d49110)
(cherry picked from commit 0e8890a425)
Approved by: so
Security: FreeBSD-EN-25:12.efi