[cloud] Display build architecture in AWS EC2

[cloud] Display build architecture in AWS EC2

On some newer (7th and 8th generation) instance types, the 32-bit
build of iPXE cannot access PCI configuration space since the ECAM is
placed outside of the 32-bit address space.  The visible symptom is
that iPXE fails to detect any network devices.

The public AMIs are all now built as 64-bit binaries, but there is
nothing that prevents the building and importing of a 32-bit AMI.
There are still potentially valid use cases for 32-bit AMIs (e.g. if
planning to use the AMI only for older instance types), and so we
cannot sensibly prevent this error at build time.

Display the build architecture as part of the AWS EC2 embedded script,
to at least allow for easy identification of this particular failure
mode at run time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>