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
(cherry picked from commit cd2b55e536dc806f9358f71db438dd9c246cdb14)

Squashed with

ralloc: don't leave out the alignment factor

Experimentation shows that without alignment factor gcc and clang choose
a factor of 16 even on IA-32, which doesn't match what malloc() uses (8).
The problem is it makes gcc assume the pointer is 16 byte aligned, so
with -O3 it starts using aligned SSE instructions that later fault,
so always specify a suitable alignment factor.

Cc: Jonas Pfeil <pfeiljonas@gmx.de>
Fixes: cd2b55e5 "ralloc: Make sure ralloc() allocations match malloc()'s alignment."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100049
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Tested by: Mike Lothian <mike@fireburn.co.uk>
Tested by: Jonas Pfeil <pfeiljonas@gmx.de>

(cherry picked from commit ff494fe999510ea40e3ed5827e7818550b6de126)
diff --git a/src/util/ralloc.c b/src/util/ralloc.c
index 980e4e4..b08df9d 100644
--- a/src/util/ralloc.c
+++ b/src/util/ralloc.c
@@ -51,7 +51,20 @@
 #define CANARY 0x5A1106
-struct ralloc_header
+/* Align the header's size so that ralloc() allocations will return with the
+ * same alignment as a libc malloc would have (8 on 32-bit GLIBC, 16 on
+ * 64-bit), avoiding performance penalities on x86 and alignment faults on
+ * ARM.
+ */
+#ifdef _MSC_VER
+ __declspec(align(8))
+#elif defined(__LP64__)
+ __attribute__((aligned(16)))
+ __attribute__((aligned(8)))
+   ralloc_header
 #ifdef DEBUG
    /* A canary value used to determine whether a pointer is ralloc'd. */