[bitcoin/bitcoin] Always create signatures with Low R values (#13666)

Clarification request:

Ref: https://tools.ietf.org/html/rfc6979#section-3.2

In `3.2 d and f`. it states:

«`
d:
K = HMAC_K(V || 0x00 || int2octets(x) || bits2octets(h1))
f:
K = HMAC_K(V || 0x01 || int2octets(x) || bits2octets(h1))
«`

but in the case where r is >= 2^255 we must re-try the RFC6979 k generation this time with the extra_entropy set to 1 (since when counter is set to 0, extra_entropy becomes nullptr, and the `WriteLE32(extra_entropy, ++counter)` is using ++ beforehand) and the following is done. shown below:

«`
d:
K = HMAC_K(V || 0x00 || int2octets(x) || bits2octets(h1) || int2octets(extra_entropy))
f:
K = HMAC_K(V || 0x01 || int2octets(x) || bits2octets(h1) || int2octets(extra_entropy))
«`

@sipa just checking, but is the appended buffer for extra_entropy big endian?