[3.14] gh-137185: Fix _Py_DumpStack() async signal safety (gh-137187) (gh-137206)

Call backtrace() once when installing the signal handler to ensure that
libgcc is dynamically loaded outside the signal handler.

This fixes a "signal-unsafe call inside of a signal" TSan error from
test_faulthandler.test_enable_fd.
(cherry picked from commit 11a8652e25)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Python/traceback.c

@@ -1327,3 +1327,13 @@ _Py_DumpStack(int fd)
     PUTS(fd, "  <cannot get C stack on this system>\n");
 }
 #endif
+
+void
+_Py_InitDumpStack(void)
+{
+#ifdef CAN_C_BACKTRACE
+    // gh-137185: Call backtrace() once to force libgcc to be loaded early.
+    void *callstack[1];
+    (void)backtrace(callstack, 1);
+#endif
+}