Replace ___constant_swab32() with gcc's __builtin_bswap32

http://www.bitcoin.org/smf/index.php?topic=1925.msg34827#msg34827

Credit: lfm

miner.h

@@ -22,15 +22,9 @@
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
 #endif
 
-#define ___constant_swab32(x) ((uint32_t)(                       \
-        (((uint32_t)(x) & (uint32_t)0x000000ffUL) << 24) |            \
-        (((uint32_t)(x) & (uint32_t)0x0000ff00UL) <<  8) |            \
-        (((uint32_t)(x) & (uint32_t)0x00ff0000UL) >>  8) |            \
-        (((uint32_t)(x) & (uint32_t)0xff000000UL) >> 24)))
-
 static inline uint32_t swab32(uint32_t v)
 {
-	return ___constant_swab32(v);
+	return __builtin_bswap32(v);
 }
 
 extern bool opt_debug;