[X86] Add support for `-mharden-sls=[none|all|return|indirect-jmp]`

The patch addresses the feature request from https://github.com/ClangBuiltLinux/linux/issues/1633. The implementation borrows a lot from aarch64.

Reviewed By: nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D126137