Meta: Load the x86_64 prekernel executable in debug-kernel.sh

debug-kernel.sh currently attempts to load "Prekernel64" as the
x86_64 prekernel executable.

However, the build system now produces "kernel_x86-64" instead,
causing GDB to fail to load the prekernel executable during
startup.

Load the generated executable so that prekernel debugging works
without manually adding the prekernel symbol file.

Meta/debug-kernel.sh

@@ -40,7 +40,7 @@ fi
 #
 if [ "$SERENITY_ARCH" = "x86_64" ]; then
     gdb_arch=i386:x86-64
-    prekernel_image=Prekernel64
+    prekernel_image=kernel_x86-64
     kernel_image=Kernel_shared_object
     kernel_base=0x2000200000
 elif [ "$SERENITY_ARCH" = "aarch64" ]; then