mesa: Use typecast in BITFIELD64_BIT macro.

Unsigned long is 32bit on several platforms (e.g., Windows), yielding
1UL << 32 to be zero.

Note that BITFIELD64_BIT result is often assigned to variables of type
GLbitfield, instead of GLbitfield64. That's probably wrong and should be
addressed in a later change.

src/mesa/main/mtypes.h

@@ -85,11 +85,7 @@
 typedef GLuint64 GLbitfield64;
 
 /** Set a single bit */
-#ifdef __cplusplus
-#define BITFIELD64_BIT(b)      (1UL << (b))
-#else
-#define BITFIELD64_BIT(b)      (1ULL << (b))
-#endif
+#define BITFIELD64_BIT(b)      ((GLbitfield64)1 << (b))
 
 
 /**