[flang] Adjust supported compilers statement in F18 readme

This patch is an update of the readme according to compilations done and
also information given in flang dev mailing list.
[F18] Which compilers configurations f18 have being tested/built?

Signed-off-by: Caroline Concatto <caroline.concatto@arm.com>

Original-commit: flang-compiler/f18@3749b463dc6d4e2847c272bf2f6b575478391a2a
Reviewed-on: https://github.com/flang-compiler/f18/pull/1087