[project] Clean up src/ tree

A long overdue cleanup of the src/ tree.

- Moved src/drivers to src/user because it contains more than drivers
- Removed src/drivers/ahci because it's unused - will restore it when I
  make a real AHCI driver
- Removed unused src/tools
- Moved kernel.ld (the only used file under src/arch) to src/kernel for
  now, if/when there's a multi-platform effort that should be figured
  out as part of it
- Removed the rest of the unused src/arch
- Renamed 'fb' to 'drv.uefi_fb' and 'nulldrv' to 'testapp'

configs/debug.toml

@@ -79,7 +79,7 @@ cxxflags = [
 
 ldflags = [
 	"${ldflags}",
-	"-T", "${source_root}/src/arch/x86_64/kernel.ld",
+	"-T", "${source_root}/src/kernel/kernel.ld",
     "-g",
     "-nostdlib",
     "-Bstatic",