base, sim, arch: Fix clang 5.0 warnings

base, sim, arch: Fix clang 5.0 warnings

Compiling gem5 with recent version of clang (4 and 5) triggers
warnings that are treated as errors:

  * Global templatized static functions result in a warning if they
    are not used. These should either be declared as static inline or
    without the static identifier to avoid the warning.

  * Some templatized classes contain static variables. The
    instantiated versions of these variables / templates need to be
    explicitly declared to avoid a compiler warning.

Change-Id: Ie8261144836e94ebab7ea04ccccb90927672c257
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3420
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>