[Flang] Add user option -funderscoring/-fnounderscoring to control trailing...

[Flang] Add user option -funderscoring/-fnounderscoring to control trailing underscore added to external names

This patch adds user option -funderscoring/-fnounderscoring  to control the trailing underscore being appended to external names (e.g. procedure names, common block names). The option in gfortran is documented in https://gcc.gnu.org/onlinedocs/gfortran/Code-Gen-Options.html.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D140795