Begin porting string performance tests to benchtests

Begin porting string performance tests to benchtests

This is the initial support for string function performance tests,
along with copying tests for memcpy and memcpy-ifunc as proof of
concept.  The string function benchmarks perform operations at
different alignments and for different sizes and compare performance
between plain operations and the optimized string operations.  Due to
this their output is incompatible with the function benchmarks where
we're interested in fastest time, throughput, etc.

In future, the correctness checks in the benchmark tests can be
removed.  Same goes for the performance measurements in the
string/test-*.