Add malloc micro benchmark

Add a malloc micro benchmark to enable accurate testing of the
various paths in malloc and free.  The benchmark does a varying
number of allocations of a given block size, then frees them again.

It tests 3 different scenarios: single-threaded using main arena,
multi-threaded using thread-arena, main arena with SINGLE_THREAD_P
false.

	* benchtests/Makefile: Add malloc-simple benchmark.
	* benchtests/bench-malloc-simple.c: New benchmark.