[x86_64] Add support for compilation as an x86_64 binary

Currently the only supported platform for x86_64 is EFI.

Building an EFI64 gPXE requires a version of gcc that supports
__attribute__((ms_abi)).  This currently means a development build of
gcc; the feature should be present when gcc 4.4 is released.

In the meantime; you can grab a suitable gcc tree from

  git://git.etherboot.org/scm/people/mcb30/gcc/.git

src/arch/x86/Makefile

@@ -0,0 +1,8 @@
+# Include common x86 headers
+#
+CFLAGS		+= -Iarch/x86/include
+
+# x86-specific directories containing source files
+#
+SRCDIRS		+= arch/x86/core
+SRCDIRS 	+= arch/x86/interface/efi