[X86] Use PC-relative mode for the kernel code model

Summary:
The Linux kernel uses PC-relative mode, so allow that when the code model is
"kernel".

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits, kees, nickdesaulniers

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60643

llvm-svn: 358343