Revert "headers: avoid bareword attributes" for clang

Revert "headers: avoid bareword attributes" for clang

This reverts most of commit 979d467ff6e39ee5c52cf1aac7a6c9c63058141c.

We cannot avoid some bareword attributes until clang is fixed to
properly support __-decorated attributes; see this bug:
https://bugs.llvm.org/show_bug.cgi?id=34319

The macros in question expand to the empty string under gcc, so
only compilation under clang is affected, and since clang has the
bug, the obvious solution is to roll back the changes, and document
the issue.

Signed-off-by: Eric Blake <eblake@redhat.com>