[llvm] Disable xar on macOS 12.0 and later

The xar file format has been deprecated since macOS 12.0, causing a
warning to be printed:

  warning: 'xar_open' is deprecated: first deprecated in macOS 12.0 -
  xar is a deprecated file format and should not be used.

Disable xar support when the macosx deployment target is greater or
equal to 12.0.

Differential revision: https://reviews.llvm.org/D112849