crypto: cleanup sha256 build

Summary:
Rather than appending all possible cpu variants to all targets, create a
convenience variable that encompasses all.

This is a backport of core PR13408

Depends on D1847

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1850