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/Arch/aarch64/linker.ld

@@ -31,6 +31,13 @@ SECTIONS
         *(.text*)
     } :text
 
+    .driver_init ALIGN(4K) : AT (ADDR(.driver_init))
+    {
+        driver_init_table_start = .;
+        *(.driver_init)
+        driver_init_table_end = .;
+    } :text
+
     .rodata ALIGN(4K) : AT (ADDR(.rodata) - KERNEL_MAPPING_BASE)
     {
         start_heap_ctors = .;

Kernel/Arch/riscv64/linker.ld

@@ -39,6 +39,13 @@ SECTIONS
         end_of_kernel_text = .;
     } :text
 
+    .driver_init ALIGN(4K) : AT (ADDR(.driver_init))
+    {
+        driver_init_table_start = .;
+        *(.driver_init)
+        driver_init_table_end = .;
+    } :text
+
     .rodata ALIGN(4K) :
     {
         start_heap_ctors = .;

Kernel/Arch/x86_64/linker.ld

@@ -38,6 +38,13 @@ SECTIONS
         *(.text*)
     } :text
 
+    .driver_init ALIGN(4K) : AT (ADDR(.driver_init))
+    {
+        driver_init_table_start = .;
+        *(.driver_init)
+        driver_init_table_end = .;
+    } :text
+
     .unmap_after_init ALIGN(4K) : AT (ADDR(.unmap_after_init))
     {
         start_of_unmap_after_init = .;

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;