gallium: use a packed enum to make pipe_prim_mode 1-byte large with __GNUC__

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12480>

src/util/macros.h

@@ -327,6 +327,13 @@ do {                       \
 #define ATTRIBUTE_NOINLINE
 #endif
 
+/* Use as: enum name { X, Y } ENUM_PACKED; */
+#if defined(__GNUC__)
+#define ENUM_PACKED __attribute__((packed))
+#else
+#define ENUM_PACKED
+#endif
+
 
 /**
  * Check that STRUCT::FIELD can hold MAXVAL.  We use a lot of bitfields