Add QBT_APP_64BIT define

Now we use QT_POINTER_SIZE to detect CPU bitness, it has the advantage of applicable to all CPU
architectures not limiting to x86 arch.

src/base/bittorrent/session.cpp

@@ -2777,7 +2777,7 @@ int Session::diskCacheSize() const
 {
     int size = m_diskCacheSize;
     // These macros may not be available on compilers other than MSVC and GCC
-#if defined(__x86_64__) || defined(_M_X64)
+#ifdef QBT_APP_64BIT
     size = qMin(size, 4096);  // 4GiB
 #else
     // When build as 32bit binary, set the maximum at less than 2GB to prevent crashes
@@ -2789,7 +2789,7 @@ int Session::diskCacheSize() const
 
 void Session::setDiskCacheSize(int size)
 {
-#if defined(__x86_64__) || defined(_M_X64)
+#ifdef QBT_APP_64BIT
     size = qMin(size, 4096);  // 4GiB
 #else
     // allocate 1536MiB and leave 512MiB to the rest of program data in RAM

src/base/global.h

@@ -31,6 +31,10 @@
 #include <type_traits>
 #include <QtGlobal>
 
+#if (QT_POINTER_SIZE == 8)
+#define QBT_APP_64BIT
+#endif
+
 const char C_TORRENT_FILE_EXTENSION[] = ".torrent";
 
 template <typename T>