Support for building PAPI via a submodule (#41)

* Enable building PAPI via submodule

* Miscellaneous fixes

- Use TIMEMORY_PAPI_ARRAY_SIZE in backtrace
- remove pthread_gotcha init from fork_gotcha::configure
- fix HSA OnLoad called during before tooling init

* PAPI array size + PAPI.cmake updates

- updated timemory submodule with PAPI updates
- fix for backtrace _hw_cnt_labels

* Disable OMPT for focal

* format

.gitmodules

@@ -16,3 +16,6 @@
 [submodule "external/kokkos"]
 	path = examples/lulesh/external/kokkos
 	url = https://github.com/kokkos/kokkos.git
+[submodule "external/papi"]
+	path = external/papi
+	url = https://bitbucket.org/icl/papi.git