arch-arm: clang compilation fixes

arch-arm: clang compilation fixes

Fix 1: std::pair constructor is not a constexpr in clang implementation
Fix 2: static const templates need to be defined in their
       translation unit

Change-Id: I3d9b9b47e1d148e44555f40e2e2aeff06707ff8f
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14617
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>