[clang-cl] Fix test that shouldn't be running on non-x86

The clang-cl test required x86-registered-target but it defaulted to the
host's triple and AArch64 still doesn't support COFF, so the test failed.

The triple was "aarch64-pc-windows-msvc18.0.0" with ObjectFormat equals
llvm::Triple::COFF, failing assertion:

Assertion `(TT.isOSBinFormatELF() || TT.isOSBinFormatMachO()) &&
          "Only expect Darwin and ELF targets"

in AArch64MCTargetDesc.cpp:78.

Making the test only run on Windows hosts obviously fixes the problem.

llvm-svn: 284749