[CMAKE] Improve FindGMP

Summary:
Build an imported target and allow to filter on version (unused as I
don't know what is the minimum required).

Test Plan:

cmake -GNinja .. -DSECP256K1_ENABLE_BIGNUM=ON
ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5449