ralloc: Make sure ralloc() allocations match malloc()'s alignment.

ralloc: Make sure ralloc() allocations match malloc()'s alignment.

The header of ralloc needs to be aligned, because the compiler assumes
that malloc returns will be aligned to 8/16 bytes depending on the
platform, leading to degraded performance or alignment faults with ralloc.

Fixes SIGBUS on Raspberry Pi at high optimization levels.

This patch is not perfect for MSVC, as maybe in the future the alignment
for the most demanding data type might change to more than 8.

v2: Commit message reword/typo fix, and add a bigger explanation in the
    code (by anholt)

Signed-off-by: Jonas Pfeil <pfeiljonas@gmx.de>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: mesa-stable@lists.freedesktop.org