Implement UEFI functionality in Forth

This is pretty cool. We can now print string and draw rectangles using UEFI,
with words that are defined in Forth. See example.f (which uses uefi.f) for an
example of how to draw a rectangle.

This removes support for the Linux backend that was recently merged in 9b08dec,
because it doesn't really make sense to maintain this backend if we're going to
be defining Forth words that only work on UEFI anyway.