ext: Build googlemock with googletest

googlemock needs to be built together with googletest. This patch
is enabling it by doing the following things

* Moving googletest and googlemock under the same top level directory,
so that we have a single SConscript for building them.
This also matches googletest directory structure:
googletest
|_googletest
|_googlemock

* Adding a shared SConscript which is building gmock and gtest as libgtest
* Removing googletest/SConscript

Change-Id: I88accbdd2e80fb47efaa17819233371fbad6acaa
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14696
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>