ext: Upgrade PyBind11 to version 2.2.1

This upgrade is necessary for pybind to build with GCC 7.2.

We still need to add the patch for stl.h. MSC_FULL_VER change is no longer
needed.
See https://gem5-review.googlesource.com/c/public/gem5/+/2230

Change-Id: I806729217d022070583994c2dfcaa74476aef30f
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/5801
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

ext: Add pybind rev f4b81b3

Change-Id: I52e4fc9ebf2f59da57d8cf8f3e37cc79598c2f5f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2229
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Pierre-Yves Péneau <pierre-yves.peneau@lirmm.fr>