Fix codemodels.c test case (only test mcmodel=medium on X86).

aarch64 testing is broken because "medium" is not a valid
code-model on aarch64, and codemodels.c tests that.  This fixes
that problem by adding "-triple x86_64-unknown-linux-gnu" to the
test with "-mcode-model moedium".

llvm-svn: 342812