C++ filesystem and link to dl library in test exe (#29)

* Add lib/rocprofiler-register/details/filesystem.hpp

- this provides backwards compatibility for compiler which do not have <filesystem>
  - use <experimental/filesystem> when not available

* Update tests/CMakeLists.txt

- test executables link to dl library (via CMAKE_DL_LIBS)

* Update CI workflow

- include ubuntu-20.04 jobs

.github/workflows/continuous-integration.yml

@@ -32,6 +32,14 @@ jobs:
             compiler: 'clang-15'
             ci-args: '--linter clang-tidy'
             ci-tag: '-clang-tidy'
+          - runner: 'ubuntu-20.04'
+            compiler: 'gcc-9'
+            ci-args: ''
+            ci-tag: ''
+          - runner: 'ubuntu-20.04'
+            compiler: 'clang-10'
+            ci-args: ''
+            ci-tag: ''
         #   - runner: 'ubuntu-latest'
         #     compiler: 'gcc-12'
         #     ci-args: '--memcheck ThreadSanitizer'