Add support for clang compiler attributes

Add support for clang compiler attributes

Rearrange the way that we define compiler attributes so that we
can define them for clang, too. Don't assume that clang will support
all of the attributes that gcc does, so split them up into separate
sections.

Change-Id: Ic6233f5c63a07b3ef99a85ae3be9e45e28387127
Reviewed-on: http://gerrit.openafs.org/2135
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>