perf ftrace: Use sig_atomic_t to avoid UB

Use sig_atomic_t for a variable written to in a signal handler and
read elsewhere. This is undefined behavior as per:

  https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlersSigned-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Bayduraev <alexey.v.bayduraev@linux.intel.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20221024181913.630986-5-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -36,8 +36,8 @@
 
 #define DEFAULT_TRACER  "function_graph"
 
-static volatile int workload_exec_errno;
-static bool done;
+static volatile sig_atomic_t workload_exec_errno;
+static volatile sig_atomic_t done;
 
 static void sig_handler(int sig __maybe_unused)
 {