[flang] Single entry point for GET_ENVIRONMENT_VARIABLE

This patch refactors the runtime support for GET_ENVIRONMENT_VARIABLE
to have a single entry point instead of 2. It also updates lowering
accordingly.

This makes it easier to handle dynamically optional arguments.
See also https://reviews.llvm.org/D118777

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D142489