Add a way to customize how logging is done.

Each architecture may have a different method to enable logging. Now
that can be customized with a function passed to the init_logger
function.

Also, provide a minimal x86_64 implementation.

This is the first commit where you can see logging coming from the log
crate.

Signed-off-by: Wren Turkal <wt@penguintechs.org>