Math runtime library in LLVM Flang

Hi,

I've opened a new issue on GitHub to discuss Math runtime libs in *LLVM Flang* [1]. I just wanted to advertise it here for better visibility. I hope that you don't mind that this discussion is happening on GitHub rather than here.

*A tiny bit of context*
In one of the recently reported issues on GitHub we've realised that `flang-new` is not finding various Math symbols. These symbols can be provided by linking with `pgmath`, i.e. an external dependency.

IIUC, in `bbc` one can switch between the following two options:
   * load Math routines from `pgmath` (default), or
   * use Math routines available in LLVM (optional).
This is available through the `--math-runtime` option in `bbc`.

What should be the behaviour of `flang-new` here?

Thank you,
-Andrzej

[1] https://github.com/flang-compiler/f18-llvm-project/issues/1377
[2] https://github.com/flang-compiler/f18-llvm-project/issues/1373