Implement SHA Hash for PowerPC

[edelsohn]

Implementing SHA hash for PowerPC architecture in HACL* by adding the following modules:

module Vale.PPC64LE.InsSha
Vector SHA-256 Sigma instruction:
Vector SHA-256 Sigma Word

Code.Crypto.Sha.PPC64LE modules for SHA-256 function:
Update state
Digest calculation

[edelsohn]

Pull request created
project-everest#54

[edelsohn]

Updated pull request landed
hacl-star/hacl-star#609