Kernel/USB: Add driver_init section

At any one given time, there can be an abitrary number of USB drivers in
the system. The way driver mapping works (i.e, a device is inserted, and
a potentially matching driver is probed) requires us to have
instantiated driver objects _before_ a device is inserted. This leaves
us with a slight "chicken and egg" problem. We cannot call the probe
function before the driver is initialised, but we need to know _what_
driver to initialise.

This section is designed to store pointers to functions that are called
during the last stage of the early `_init` sequence in the Kernel. The
accompanying macro in `USBDriver` emits a symbol, based on the driver
name, into this table that is then automatically called.

This way, we enforce a "common" driver model; driver developers are not
only required to write their driver and inherit from `USB::Driver`, but
are also required to have a free floating init function that registers
their driver with the USB Core.

Kernel/Bus/USB/Drivers/USBDriver.h

@@ -12,6 +12,10 @@
 
 namespace Kernel::USB {
 
+using DriverInitFunction = void (*)();
+#define USB_DEVICE_DRIVER(driver_name) \
+    DriverInitFunction driver_init_function_ptr_##driver_name __attribute__((section(".driver_init"), used)) = &driver_name::init
+
 class Device;
 struct USBDeviceDescriptor;
 class USBInterface;