[test] Include build architecture in test suite banner

[test] Include build architecture in test suite banner

The test suites for the various architectures are often run back to
back, and there is currently nothing to visually distinguish one test
run from another.

Include the architecture name within the self-test startup banner, to
aid in visual identification of test results.

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