Temporarily disable LLVM 12 support (for 1.6 release)

1.5 docs updates

1.5 docs updates

Fix some compiler warnings

Slight tweaks of CHANGES and rel.notes.

Merge branch 'release_1_4'

Update pocl version to 1.4

Revert "Update pocl git master to 1.4-pre"

This reverts commit 7407355.
This was accidentally merged into the release branch

Revert "Update pocl git master to 1.4-pre"

This reverts commit 7407355.
This was accidentally merged into the release branch

Merge branch 'cuda-printf' of https://github.com/jrprice/pocl into re…

…lease_1_3