cmake: fix building with both sha256 and sha512 in unity mode

[pps83]

• Ch and Maj are also defined in curl_sha512_256.c.
• Avoid defining short defines like R, S

[vszakats]

What do you think of also prefixing Ch and Maj in curl_sha512_256.c? Mainly for good measure/symmetry and to avoid bumping into this when adding a future hash.

[pps83]

What do you think of also prefixing Ch and Maj in curl_sha512_256.c? Mainly for good measure/symmetry and to avoid bumping into this when adding a future hash.

updated

[jay]

Thanks