title: The clang compiler does not support a feature it reportedly supports
body: <p>I'm trying to tailor compiler flags of clang-11 to the <code>apple-m1</code> CPU, which clang-11 doesn't know about yet.</p>
  <p>The output of <code>/usr/bin/clang -E - -mcpu=apple-m1 -###</code> on macOS outputs a command with flags like these in it:  <code>&quot;-target-feature&quot; &quot;+zcz&quot;</code></p>
  <p>From that you can infer the following features of the CPU:</p>
  <pre><code>armv8.5a+fp-armv8+neon+crc+crypto+dotprod+fp16fml+ras+lse+rdm+rcpc+zcm+zcz+fullfp16+sm4+sha3+sha2+aes
  </code></pre>
  <p>However, out of these, <code>+fp-armv8+neon+zcm+zcz+fullfp16</code> are not recognised to be valid by any clang compiler:</p>
  <pre><code>$ cc -march=armv8.5a+zcz test.c 
  clang-11: error: the clang compiler does not support '-march=armv8.5a+zcz'
  </code></pre>
  <p>How can I tell clang to optimise for those target flags?</p>