Made it possible to build the bootloader when targetting x86_64.

* x86_64 is using the existing *_ia32 boot platforms.
* Special flags are required when compiling the loader to get GCC to compile
  32-bit code. This adds a new set of rules for compiling boot code rather
  than using the kernel rules, which compile using the necessary flags.
* Some x86_64 private headers have been stubbed by #include'ing the x86
  versions. These will be replaced later.

headers/private/system/arch/x86_64/arch_config.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright 2004, Alex Smith, [email protected].
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _KERNEL_ARCH_x86_64_CONFIG_H
+#define _KERNEL_ARCH_x86_64_CONFIG_H
+
+#define FUNCTION_CALL_PARAMETER_ALIGNMENT_TYPE  unsigned long
+
+#define STACK_GROWS_DOWNWARDS
+
+//#define ATOMIC_FUNCS_ARE_SYSCALLS
+//#define ATOMIC64_FUNCS_ARE_SYSCALLS
+
+#endif /* _KERNEL_ARCH_x86_64_CONFIG_H */