perf test: Add 'leafloop' test workload

The leafloop workload is to run an infinite loop in the test_leaf
function.  This is needed for the ARM fp callgraph test to verify if it
gets the correct callchains.

  $ perf test -w leafloop

Committer notes:

Add a:

  -U_FORTIFY_SOURCE

to the leafloop CFLAGS as the main perf flags set it and it requires
building with optimization, and this new test has a -O0.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zhengjun Xing <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221116233854.1596378-6-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>