Fix #148: ARM: sadc crashes because of unaligned memory accesses

Fix #148: ARM: sadc crashes because of unaligned memory accesses

Use posix_memalign() with 16 bytes alignment request instead of
realloc() to guarantee 16 byte alignment requested by several
sysstat's structures.

Note: 16 byte alignment is no longer needed by sysstat's
structures. Yet removing the corresponding attributes
(__attribute__ ((aligned (16)))) for example in rd_stats.h
would imply a format change for sar/sadc binary data files.
These attributes will be removed when work is done to make sa binary
datafiles portable (see #135).

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>