[flang] Support GNU extensions IARGC and GETARG in runtime

The GNU extension intrinsic IARGC is equivalent to
COMMAND_ARGUMENT_COUNT, and the GETARG is similar to
GET_COMMAND_ARGUMENT, but with less arguments. Reuse the runtime of
COMMAND_ARGUMENT_COUNT and GET_COMMAND_ARGUMENT for IARGC and GETARG.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D133558