[flang] Fix build when examples are disabled
Without this change I get a build error due to the missing Bye target
when I configure my build with -DLLVM_INCLUDE_EXAMPLES=OFF.
This check for LLVM_BUILD_EXAMPLES matches the checks in llvm and lld.
Reviewed By: mgorny
Pull Request: https://github.com/llvm/llvm-project/pull/137908
