Format preprocessor macros more consistently

Within the libjpeg API code, it seems to be more the convention than not
to separate the macro name and value by two or more spaces, which
improves general readability.  Making this consistent across all of
libjpeg-turbo is less about my individual preferences and more about
making it easy to automatically detect variations from our chosen
formatting convention.  I intend to release the script I'm using to
validate this stuff, once it matures and stabilizes a bit.

jcarith.c

@@ -63,8 +63,8 @@ typedef arith_entropy_encoder *arith_entropy_ptr;
  * in the lower bits (mask 0x7F).
  */
 
-#define DC_STAT_BINS 64
-#define AC_STAT_BINS 256
+#define DC_STAT_BINS  64
+#define AC_STAT_BINS  256
 
 /* NOTE: Uncomment the following #define if you want to use the
  * given formula for calculating the AC conditioning parameter Kx