devel/oclgrind: update to 21.10

Changelog: https://github.com/jrprice/Oclgrind/releases/tag/v21.10

Update LLVM dependency to LLVM12 - minimum LLVM11 is required upstream.

Remove powerpc special casing in-line with comment (LLVM >9)

Add pkg-message that procfs(5) is required.

Clean pkg-descr whitespace.