[crypto] Extract bigint_reduce_supremum() from bigint_mod_exp()

Calculating the Montgomery constant (R^2 mod N) is done in our
implementation by zeroing the double-width representation of N,
subtracting N once to give (R^2 - N) in order to obtain a positive
value, then reducing this value modulo N.

Extract this logic from bigint_mod_exp() to a separate function
bigint_reduce_supremum(), to allow for reuse by other code.

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

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