Add option to define ranges sets for benchmarks

This patch add an option to use the pre-define set of ranges along
with macros AT_ONEBUF_MANUAL_ALIGN_* and AT_TWOBUF_MANUAL_ALIGN1_*.

The size argument can be either a number or a string representing
the sets of values:

  * SMALL for values between 1 and 256.
  * MEDIUM for values between 512 and 128KB.
  * LARGE for values between 256KB and 2048KB.

Test: Ran new string test suite with a single bionic iteration.
Change-Id: Ieda81ee9a5019991b0b2f97d4ca3a237127c5848