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>

src/arch/x86_64/start.rs

@@ -84,7 +84,7 @@ pub unsafe extern fn kstart(args_ptr: *const KernelArgs) -> ! {
         KERNEL_SIZE.store(kernel_size, Ordering::SeqCst);
 
         // Initialize logger
-        log::init_logger();
+        log::init_logger(|r| println!("{}:{} -- {}", r.target(), r.level(), r.args()));
 
         info!("Redox OS starting...");
         println!("Kernel: {:X}:{:X}", kernel_base, kernel_base + kernel_size);