Fix codemodels.c test case (only test mcmodel-kernel on x86)

A recent commit I made broke aarch64 testing, because "kernel"
apparently is not a valid code-model on aarch64, and one of my tests
tested that. This fixes the problem (hopefully) by adding "-triple
x86_64-unknown-linux-gnu" to the test build with "-mcodel-model
kernel".

Differential Revision: https://reviews.llvm.org/D52383

llvm-svn: 342789