[X86] Correct type for argument to clflushopt intrinsic.

Summary: According to Intel docs this should take void const *. We had char*. The lack of const is the main issue.

Reviewers: RKSimon, zvi, igorb

Reviewed By: igorb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38782

llvm-svn: 315470