[cmake] do not set execution permission to regular files.

some regular files(e.g. files have no shebang and no execute
bit in source dir) are wrongly assigned an execution permission,
such as scanview.css and ear.c from libscanbuild, which is
unnecessary and introduces warnings in some envs.

Reviewed By: MaskRay, phosek

Differential Revision: https://reviews.llvm.org/D133308