New make target to only build benchmark binaries

New make target to only build benchmark binaries

For situations where we are cross-building or where we want to avoid
building on the target system, we want a way to only build benchmarks
and then copy them over to the target system to run them.  I have also
added a simple enhancement for the 'bench' target where all benchmark
binaries are built and then the benchmarks executed.

Tested on arm.

	Makefile.in (bench-build): New target.
	Rules (PHONY): Add bench-build target.
	benchtests/Makefile (bench): Depend on bench-build.
	(bench-build): New target.