Removing the 'byteswap.h' header which is not portable

Incorporating code review changes:
https://github.com/google/upb/pull/151#discussion_r257529497

upb/pb/varint.int.h

@@ -9,7 +9,6 @@
 #include <assert.h>
 #include <stdint.h>
 #include <string.h>
-#include <byteswap.h>
 #include "upb/upb.h"
 
 #ifdef __cplusplus
@@ -25,6 +24,18 @@ extern "C" {
  * descriptor type (upb_descriptortype_t). */
 extern const uint8_t upb_pb_native_wire_types[];
 
+UPB_INLINE uint64_t byteswap64(uint64_t val)
+{
+  return ((((val) & 0xff00000000000000ull) >> 56)
+    | (((val) & 0x00ff000000000000ull) >> 40)
+    | (((val) & 0x0000ff0000000000ull) >> 24)
+    | (((val) & 0x000000ff00000000ull) >> 8)
+    | (((val) & 0x00000000ff000000ull) << 8)
+    | (((val) & 0x0000000000ff0000ull) << 24)
+    | (((val) & 0x000000000000ff00ull) << 40)
+    | (((val) & 0x00000000000000ffull) << 56));
+}
+
 /* Zig-zag encoding/decoding **************************************************/
 
 UPB_INLINE int32_t upb_zzdec_32(uint32_t n) {
@@ -132,7 +143,7 @@ UPB_INLINE uint64_t upb_vencode32(uint32_t val) {
   UPB_ASSERT(bytes <= 5);
   memcpy(&ret, buf, bytes);
 #ifdef UPB_BIG_ENDIAN
-  ret = bswap_64(ret);
+  ret = byteswap64(ret);
 #endif
   UPB_ASSERT(ret <= 0xffffffffffU);
   return ret;