patch 8.2.0739: incomplete profiling when exiting because of a dealy signal

Problem:    Incomplete profiling when exiting because of a dealy signal.
Solution:   Call __gcov_flush() if available.
diff --git a/src/os_unix.c b/src/os_unix.c
index b5ae9f8..aea3d5a 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3312,6 +3312,10 @@
     }
 }
 
+#ifdef USE_GCOV_FLUSH
+extern void __gcov_flush();
+#endif
+
     void
 mch_exit(int r)
 {
@@ -3358,6 +3362,12 @@
     }
     out_flush();
     ml_close_all(TRUE);		// remove all memfiles
+
+#ifdef USE_GCOV_FLUSH
+    // Flush coverage info before possibly being killed by a deadly signal.
+    __gcov_flush();
+#endif
+
     may_core_dump();
 #ifdef FEAT_GUI
     if (gui.in_use)