Try to fix cl-options.c on bots were the default triple is non-x86 non-arm

llvmArchToWindowsSDKArch() returns "" for non-intel non-arm archs.
We're checking for "/fake/lib/" which is followed by the result
of that function -- but if that returns an empty string, then that
trailing slash isn't there. As fix, just explicitly pass a triple
that's intel or arm (I randomly chose aarch64). Since the test runs
with -###, that arch doesn't have to be in LLVM_TARGETS_TO_BUILD.