Revert "hack to unbreak check-llvm on win after D97335" in attempt for actual fix

This reverts commit 900f0761 and attempts an actual fix: All failing tests for llvm-jitlink use the `-noexec` flag. The inputs they operate on are not meant for execution on the host system. Looking e.g. at the MachO_test_harness_harnesss.s test, llvm-mc generates input machine code with "x86_64-apple-macosx10.9".

My previous attempt in bbdb4c8c disabled the debug support plugin for Windows targets, but what we would actually want is to disable it on Windows HOSTS.

With the new patch here, I don't do exactly that, but instead follow the approach for the EH frame plugin and include the `-noexec` flag in the condition. It should have the desired effect when it comes to the test suite. It appears a little workaround'ish, but should work reliably for now. I will discuss the issue with Lang and see if we can do better. Thanks @thakis again for the temporary fix.