[build] Update to using pb 3

Updating the build to the new version of bonnibel. This also includes
some updates to make sure things keep working with LLVM 11.

src/drivers/nulldrv/module.toml

@@ -0,0 +1,8 @@
+name = "nulldrv"
+targets = ["user"]
+deps = ["libc"]
+sources = [
+	"io.cpp",
+	"main.cpp",
+	"serial.cpp",
+]