valgrind: update to 3.21.0

Release Notes:
https://valgrind.org/docs/manual/dist.news.html

This improves support for the memory allocator used in musl libc 1.2.2
and later which is currently used by OpenWrt.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit d85013460d)

package/devel/valgrind/patches/100-fix_configure_check.patch

@@ -1,6 +1,6 @@
 --- a/configure.ac
 +++ b/configure.ac
-@@ -368,7 +368,7 @@ case "${host_os}" in
+@@ -364,7 +364,7 @@ case "${host_os}" in
          # Ok, this is linux. Check the kernel version
          AC_MSG_CHECKING([for the kernel version])