[Flang] Make all tests work with lit's internal shell
This patch removes REQUIRES: shell lines and updates the tests to not
need a shell, or adds REQUIRES lines for Linux where relevant. A lot of
these seem to have gotten introduced in
58c3f20bbf51f454ffb38ddb700a1d5ad374dc7b, but many are no longer
relevant as these tests no longer use shell scripts.
There was one test (missing_newline.f90) that needed to be modified to
work. Lit doesn't recognize -ne correctly for echo and the options need
to be split into separate flags.
Reviewers: clementval, Meinersbur, kiranchandramohan, klausler
Reviewed By: clementval
Pull Request: https://github.com/llvm/llvm-project/pull/156115
