[flang] Extended the flang driver options to include gfortran equivalents to...

[flang] Extended the flang driver options to include gfortran equivalents to pgf90 specific options.

Summary: Added gfortran equivalents of pgf90's -Mfixed, -Mfree, -Mextend, -Mstandard, -Munlimited and also added -fdefault-double-8

Reviewers: sscalpone, richard.barton.arm, DavidTruby, clementval, jdoerfert

Reviewed By: sscalpone, richard.barton.arm

Subscribers: sstefan1, llvm-commits, flang-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D83687