[CI][C++][Gandiva] MacOS builds are failing because of the version of the LLVM

[asfimport]

The actual version of the LLVM library inside the macOS builds is 12, but the Gandiva only supports the versions between 6 to 12.

Reporter: Anthony Louis Gotlib Ferreira / @anthonylouisbsb

Related issues:

• [C++][Gandiva] Add support for LLVM12 (is duplicated by)

_{Note: This issue was originally created as ARROW-12483. Please see the migration documentation for further details.}