Print help for -? and /? cross-plat

[ViktorHofer]

Fixes #42395

[danmoseley]

thanks @ViktorHofer

[ivdiazsa]

While trying to reproduce this issue, I found something interesting in build.ps1, in the following code snippet:

if ($help -or (($null -ne $properties) -and ($properties.Contains('/help') -or $properties.Contains('/?')))) {
  Get-Help
  exit 0
}

The $properties value is always empty in this clause, so we might as well remove those checks, and stay only with the $help one.

[ivdiazsa]

This was an accident. Pinged Dan to approve the PR again.

[ViktorHofer]

/azp run runtime

[azure-pipelines]

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