Revert "-fstack-clash-protection: Return an actual error when used on unsupported OS"

This reverts commit 4d59c8fd.
Breaks tens of thousands of tests, and had pending review comments, see
comments on https://reviews.llvm.org/D92245 (and e.g.
http://lab.llvm.org:8011/#/builders/109/builds/5236 for failures).