fix: test for arm64 architecture properly in MP3DecoderTests

on arm64/aarch64 libmpg123 generates bad data leading to the test
MP3DecoderTests.test_DecodeMP3As2PiecesMPG123_SameAs1Piece failing. this
test was excluded by testing for the TARGET_CPU_ARM64 macro. but this is
an Xcode macro defined in TargetConditionals.h. thus the test would not
be excluded when compiling on a device such as the Raspberry Pi 4 or
5. the portable way to test for arm64 architecture with clang is to
   check for __aarch64__: https://stackoverflow.com/a/41666292

Tests/MP3DecoderTests.cpp

@@ -92,7 +92,7 @@ static void runMp3TwoPieceTestWithBackend(int backend) {
     EXPECT_TRUE(cmp==0);
 }
 
-#if !TARGET_CPU_ARM64
+#if !__aarch64__
 // FIXME: Looks like MPG123 generates bad data in that test. Needs proper debugging on arm64 host.
 TEST_F(MP3DecoderTests, test_DecodeMP3As2PiecesMPG123_SameAs1Piece) {
     runMp3TwoPieceTestWithBackend(0); // mpg123