Kernel: Build the x86_64 kernel as an ELF32 executable

Multiboot only supports ELF32 executables. This changes the build
process to build an ELF32 executable which has a 32-bit entry point,
but consists of mostly 64-bit code.

Meta/debug-kernel.sh

@@ -8,9 +8,17 @@
 # remote on localhost:1234. So point our debugger there, and inform
 # the debugger which binary to load symbols, etc from.
 #
+if [ "$SERENITY_ARCH" = "x86_64" ]; then
+    gdb_arch=i386:x86-64
+    kernel_binary=Kernel64
+else
+    gdb_arch=i386:intel
+    kernel_binary=Kernel
+fi
+
 exec $SERENITY_KERNEL_DEBUGGER \
-    -ex "file $(dirname "$0")/../Build/${SERENITY_ARCH:-i686}/Kernel/Kernel" \
-    -ex 'set arch i386:intel' \
+    -ex "file $(dirname "$0")/../Build/${SERENITY_ARCH:-i686}/Kernel/$kernel_binary" \
+    -ex "set arch $gdb_arch" \
     -ex 'target remote localhost:1234' \
     -ex "source $(dirname "$0")/serenity_gdb.py" \
     "$@"