stand/efi: Add a 32-bit variant of libefi

In preparation for supporting 64-bit machines with 32-bit UEFI firmware,
add a 32-bit variant of libefi since we need to compile both the 64-bit
version and the 32-bit version at the same time.

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