Debugging: Add kernel debugging support

Introduce the 'debug-kernel' script to allow developers to
quickly attach a debugger to the QEMU debug remote. The
setting (-s) is already enabled by ./run today when using
QEMU for virtualisation.

If the system is running under QEMU, the debugger
will break in when the script is run. If you add
the -S option to QEMU it will wait for the debugger
to connect before booting the kernel. This allows
you to debug the init/boot process.

Personally I use cgdb instead of gdb, so I opted
to make the debugger used by the script customizable
via an environment variable.

This change also adds -g3 to the kernel build so that
rich debug symbols are available in the kernel binary.

Kernel/debug-kernel

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+# Set this environment variable to override the default debugger.
+#
+[ -z "$SERENITY_KERNEL_DEBUGGER" ] && SERENITY_KERNEL_DEBUGGER="gdb"
+
+# The QEMU -s option (enabled by default in ./run) sets up a debugger
+# remote on localhost:1234. So point our debugger there, and inform
+# the debugger which binary to load symbols, etc from.
+#
+$SERENITY_KERNEL_DEBUGGER \
+    -ex "file $(pwd)/kernel" \
+    -ex 'set arch i386:intel' \
+    -ex 'target remote localhost:1234'