Fix the unaligned string toLowerAscii test

Fix the unaligned string toLowerAscii test

Summary:
* The test computed nonaligned inputs but then copied them into temporary
buffers to compare different implementations. The temporary buffers
were word-aligned.
* This diff keeps the temp buffers but ensures that the data in them
keeps the original input's alignment.

Test Plan:
* Ran the test case with a modified String.cpp containing an assert to
catch unaligned reads.  The assert failed, as expected, on a copy
of the code without the fix from D1434585

Reviewed By: meyering@fb.com

Subscribers: ruibalp

FB internal diff: D1435028

Tasks: 4696800