Revise RenderScript attribute declaration

Summary:
Address post-patch comments to r272342.

- Rename the 'kernel' attribute's name
- Idiomatically restrict the attribute to the 'RenderScript' LangOpt.

Reviewers: aaron.ballman, rsmith

Subscribers: cfe-commits, srhines

Differential Revision: http://reviews.llvm.org/D21240

llvm-svn: 272438