gem5/gem5-aarch64-native: backport a patch to reduce warnings

With Python 3.9 the build produces a huge number of warnings, resulting in a
do_compile log that is 17M long.

Backport a patch from pybind11 upstream to silence this warning.

Change-Id: Ie4c71f8e5dd6c07c8be0a563778029a9859e1726
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>