patch 8.2.3770: new compiler warnings from clang-12 and clang-13

Problem:    New compiler warnings from clang-12 and clang-13.
Solution:   Adjust CI and suppress some warnings. (Ozaki Kiichi, closes #9314)
diff --git a/src/os_unix.c b/src/os_unix.c
index 32850f3..1c00f22 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -3376,7 +3376,15 @@
 }
 
 #ifdef USE_GCOV_FLUSH
-extern void __gcov_flush();
+# if (defined(__GNUC__) \
+	    && ((__GNUC__ == 11 && __GNUC_MINOR__ >= 1) || (__GNUC__ >= 12))) \
+	|| (defined(__clang__) && (__clang_major__ >= 12))
+extern void __gcov_dump(void);
+extern void __gcov_reset(void);
+#  define __gcov_flush() do { __gcov_dump(); __gcov_reset(); } while (0)
+# else
+extern void __gcov_flush(void);
+# endif
 #endif
 
     void
diff --git a/src/spellfile.c b/src/spellfile.c
index 6d6c248..46d222d 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -6576,7 +6576,7 @@
     char_u	*p;
     int		len;
     int		totlen;
-    size_t	x = 1;  // collect return value of fwrite()
+    size_t	x UNUSED = 1;  // collect return value of fwrite()
 
     if (fd != NULL)
 	put_bytes(fd, (long_u)gap->ga_len, 2);	    // <prefcondcnt>
diff --git a/src/version.c b/src/version.c
index 5aa6ce7..5db7dca 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3770,
+/**/
     3769,
 /**/
     3768,