liblzma: Make liblzma use libmd implementation of SHA256.

[delphij]

Summary

PR: 200142

[emaste]

Do we need to request an up-to-date exp-run?

[allanjude]

Thanks for cleaning this up, as noted in rS292782 the libmd version was optimized by cperciva@

Reviewed by: allanjude

[delphij]

I've requested a new exp-run with this one.

[delphij]

Remove unused sha256.c and use sed instead of awk.

[delphij]

Landed as rS362452.