[test] Add tests for 64-bit division

On a 32-bit system, 64-bit division is implemented using the libgcc
functions provided in __udivmoddi4.c etc.  Calls to these functions
are generated automatically by gcc, with a calling convention that is
somewhat empirical in nature.  Add these self-tests primarily as a
check that we are using the correct calling convention.

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