[4.2] Disable testing printing of subnormals on 32b ARM. (#16433)

[milseman]

Cherry-pick of #16433

On Darwin (and most other OSes), subnormal support is turned off for armv7, which causes these tests to fail. Longer-term, instead of gating this on architecture we may want to be able to query if subnormals are supported for a BinaryFloatingPoint type, and then we could key off of that instead.

rdar://problem/40454027

[milseman]

@stephentyrone @tbkka were there any other changes on master that didn't get cherry-picked onto 4.2? I can cherry-pick them here too

[milseman]

@swift-ci please test

[stephentyrone]

This is all I know of.

[tbkka]

I don't recall any others.

[tbkka]

FWIW, there already is a very simple test for whether subnormals are supported:

  if (Float.leastNonzeroMagnitude != Float.leastNormalMagnitude) {
    // Read this as: "The smallest Float is not the smallest Normal float"
    // ==> Float supports subnormals
  }