[crypto] Use architecture-independent bigint_is_set()

[crypto] Use architecture-independent bigint_is_set()

Every architecture uses the same implementation for bigint_is_set(),
and there is no reason to suspect that a future CPU architecture will
provide a more efficient way to implement this operation.

Simplify the code by providing a single architecture-independent
implementation of bigint_is_set().

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