ext: import GDB XML target description files for arm

The XML files were copied from the binutils-gdb source tree under
gdb/features at tag gdb-8.2-release Those XML files have a different
copyright header than the rest of binutils-gdb which allows them to be
copied into non-GPL projects.

Change-Id: I49bdeaad91ceb284c73cc0b861906ce09e44ca1d
Reviewed-on: https://gem5-review.googlesource.com/c/15256
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>