[x86] Fix isOffsetSuitableForCodeModel kernel code model offset

Offset == 0 is a valid offset for kernel code model according to the
x86_64 System V ABI. Found by inspection, no testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223383 91177308-0d34-0410-b5e6-96231b3b80d8