benchtests: Add malloc microbenchmark

benchtests: Add malloc microbenchmark

Add a microbenchmark for measuring malloc and free performance with
varying numbers of threads. The benchmark allocates and frees buffers
of random sizes in a random order and measures the overall execution
time and RSS. Variants of the benchmark are run with 1, 8, 16 and
32 threads.

The random block sizes used follow an inverse square distribution
which is intended to mimic the behaviour of real applications which
tend to allocate many more small blocks than large ones.

ChangeLog:

2014-11-05  Will Newton  <will.newton@linaro.org>

	* benchtests/Makefile: (bench-malloc): Add malloc thread
	scalability benchmark.
	* benchtests/bench-malloc-threads.c: New file.