[emacs] Use c-mode-common-hook, derive from "gnu"

Make it clear that the "llvm.org" style is deriving from "gnu" style,
and use the c-mode-common-hook instead of c-mode-hook and c++-mode-hook.

Differential Revision: http://reviews.llvm.org/D7035

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226861 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed