[perftest] Add the ability to calculate throughput (bytes/sec) for tests

Add a SetBytesProcessedPerRun() method that tests can call. Change
the memcpy() test to use that. Add a column to the output table that
shows the throughput.

Example output:

Mean Std dev Min Max Median Unit Mean Mbytes/sec Test case
27 3 25 128 27 nanoseconds 35515.502 Memcpy/1000bytes
2601 1851 2326 59162 2518 nanoseconds 36672.307 Memcpy/100000bytes

Test: "/boot/test/sys/perf-test -p"; check output for Memcpy test

Change-Id: Ib3c58ff930387946fea19134a1db5681a64465c7

[perftest] Add a performance test for memcpy()

Test: /boot/test/sys/perf-test (with -p to check the resulting times)

Change-Id: I97f667f6509c237731b38f538956c025bfdc16cb