[flang] Change the default F18_FC to gfortran

Summary: Changed default F18_FC from pgf90 to gfortran. Removed unnecessary references to pgf90 in favor of more generic naming.

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

Reviewed By: sscalpone, richard.barton.arm, clementval

Subscribers: sstefan1, tskeith, klausler, clementval, flang-commits, llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D83488