The check for unresolved symbols does not explicitly specify an output
architecture format, and so causes a warning when building an i386 EFI
binary on an x86_64 platform.  This warning is harmless, and
specifying the output architecture in multiple places is cumbersome,
so just inhibit the warning.

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