Most modern platforms use 64-bit EFI so it is useful to have a U-Boot app
that runs under that. Add a (non-functional) build for this.

Note that --whole-archive causes the gcc 9.2 linker to crash, so disable
this for now. Once this is resolved, things should work.

For now, avoid mentioning the documentation for the 64-bit app, since it
does not work.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>