[cmake][secp256k1] Check is amd64 asm is supported and if so, use it.

Summary: As per title. This makes things much faster.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1323