Fix non-SIMD alignment if void* bigger than double

When building without the SIMD extensions, memory allocations are
currently aligned to sizeof(double).  However, this may be insufficient
on architectures such as Arm Morello or 64-bit CHERI-RISC-V where
pointers require 16-byte rather than 8-byte alignment.  This patch
causes memory allocations to be aligned to
MAX(sizeof(void *), sizeof(double)) when building without the SIMD
extensions.

(NOTE: With C11 we could instead use alignof(max_align_t), but C89
compatibility is still necessary in libjpeg-turbo.)

Closes #587

jmemmgr.c

@@ -68,10 +68,13 @@ round_up_pow2(size_t a, size_t b)
  * There isn't any really portable way to determine the worst-case alignment
  * requirement.  This module assumes that the alignment requirement is
  * multiples of ALIGN_SIZE.
- * By default, we define ALIGN_SIZE as sizeof(double).  This is necessary on
- * some workstations (where doubles really do need 8-byte alignment) and will
- * work fine on nearly everything.  If your machine has lesser alignment needs,
- * you can save a few bytes by making ALIGN_SIZE smaller.
+ * By default, we define ALIGN_SIZE as the maximum of sizeof(double) and
+ * sizeof(void *).  This is necessary on some workstations (where doubles
+ * really do need 8-byte alignment) and will work fine on nearly everything.
+ * We use the maximum of sizeof(double) and sizeof(void *) since sizeof(double)
+ * may be insufficient, for example, on CHERI-enabled platforms with 16-byte
+ * pointers and a 16-byte alignment requirement.  If your machine has lesser
+ * alignment needs, you can save a few bytes by making ALIGN_SIZE smaller.
  * The only place I know of where this will NOT work is certain Macintosh
  * 680x0 compilers that define double as a 10-byte IEEE extended float.
  * Doing 10-byte alignment is counterproductive because longwords won't be
@@ -81,7 +84,7 @@ round_up_pow2(size_t a, size_t b)
 
 #ifndef ALIGN_SIZE              /* so can override from jconfig.h */
 #ifndef WITH_SIMD
-#define ALIGN_SIZE  sizeof(double)
+#define ALIGN_SIZE  MAX(sizeof(void *), sizeof(double))
 #else
 #define ALIGN_SIZE  32 /* Most of the SIMD instructions we support require
                           16-byte (128-bit) alignment, but AVX2 requires