Specialized double-SHA256 with 64 byte inputs with SSE4.1 and AVX2

Summary:

• Benchmark Merkle root computation
• Refactor SHA256 code
• Specialized double sha256 for 64 byte inputs
• Use SHA256D64 in Merkle root computation
• 4-way SSE4.1 implementation for double SHA256 on 64-byte inputs
• 8-way AVX2 implementation for double SHA256 on 64-byte inputs
• [MOVEONLY] Move unused Merkle branch code to tests
• Enable double-SHA256-for-64-byte code on 32-bit x86
• For AVX2 code, also check for AVX, XSAVE, and OS support

This is a backport of Core PR13191, PR13393 and PR13471

Depends on D1844 and D1845

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1846