[cmake] Enable assembly routines in the crypto package

Summary: This ensure we use an optimized version of sha256 if the plateform supports it.

Test Plan:

ninja bitcoind

Check the log for Using the 'sse4' SHA256 implementation

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1328