Add support for the the target attribute.

Modeled after the gcc attribute of the same name, this feature
allows source level annotations to correspond to backend code
generation. In llvm particular parlance, this allows the adding
of subtarget features and changing the cpu for a particular function
based on source level hints.

This has been added into the existing support for function level
attributes without particular verification for any target outside
of whether or not the backend will support the features/cpu given
(similar to section, etc).

llvm-svn: 239579