PROV: Fix bignum printout in text serializers
The common routine ossl_prov_print_labeled_bignum() didn't print the
BIGNUM quite the way it should. It treated the limbs in a big endian
fashion, when they are really organised in a little endian fashion.
Furthermore, we make it inherit the behaviour from the print of legacy
keys, where a number starting with the high bit set gets an extra zero
printed first.
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10891)