Add build option for the ia32 loader

In preparation for supporting 64-bit machines with 32-bit UEFI firmware,
add a build option for compiling the ia32 loader. Currently unused.

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

(cherry picked from commit fe3f792f9a2982678138e239f5e8d66e8ee71aeb)