[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_64/Makefile

@@ -0,0 +1,41 @@
+# Code size reduction.
+#
+CFLAGS		+= -fstrength-reduce -fomit-frame-pointer
+
+# Code size reduction.  gcc3 needs a different syntax to gcc2 if you
+# want to avoid spurious warnings.
+#
+CFLAGS		+= -falign-jumps=1 -falign-loops=1 -falign-functions=1
+
+# Use %rip-relative addressing wherever possible.
+#
+CFLAGS		+= -fpie
+
+# Force 64-bit code
+#
+CFLAGS		+= -m64
+ASFLAGS		+= --64
+LDFLAGS		+= -m elf_x86_64
+
+# EFI requires -fshort-wchar, and nothing else currently uses wchar_t
+#
+CFLAGS		+= -fshort-wchar
+
+# We need to undefine the default macro "i386" when compiling .S
+# files, otherwise ".arch i386" translates to ".arch 1"...
+#
+CFLAGS			+= -Ui386
+
+# x86_64-specific directories containing source files
+#
+SRCDIRS		+= arch/x86_64/prefix
+
+# Include common x86 Makefile
+#
+MAKEDEPS	+= arch/x86/Makefile
+include arch/x86/Makefile
+
+# Include platform-specific Makefile
+#
+MAKEDEPS	+= arch/x86_64/Makefile.$(PLATFORM)
+include arch/x86_64/Makefile.$(PLATFORM)