rxperf: Move into the tools directory

rxperf: Move into the tools directory

Move the 'rxperf' RX performance testing utility out of the
src/rx/test directory, and into the slightly more visible top level
src/tools/ directory

As this is the first time that rxperf has been built as part of the
default build, make a number of changes so that it will build on all
of our supported platforms.

Change-Id: Ice37e7db694dbfed34009bf76d24f1e0bf272e47
Reviewed-on: http://gerrit.openafs.org/7240
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>