[clang][X86] Update __cpuidex_conflict.c test post #126324 (#151220)
It does not look like __cpuidex builtins are being incorrectly included
in compilations for offload targets anymore, so change up the test to
assume that we are defining __cpuidex as static in cpuid.h now that
\#126324 updates the behavior of __has_builtin on offload compilations.
This ensures we are still testing that we are avoiding the conflicts
around offloading that were first pointed out in
https://reviews.llvm.org/D150646.