LLVM-Flang package (#14389)

* Add 20181226 release of pgmath

* 20190329 release of pgmath requires match for CMPLX macro.

* Add llvm-flang package for Flang fork of LLVM

* Add new and old flang releases.

* Add cmake and python dependencies.

* Update dependencies on llvm-flang and pgmath.

* Fix cmake args and change spec to reflect llvm-flang package.

* change copyright date through 2020.

* Reference Flang package more explicitly.

* More robust support for python executable.

* import os no longer needed, picked up by flake8.

* Use built-in target spec. Variant and targets follow style in main LLVM package.

* Get rid of targets list and only support one target for now.

* Sparc does not appear to be supported in Flang.

* Raise InstallError if architecture not supported.