Math: add an XFAIL for packing -1.0l to 64-bit integers on ARM Mac.

Worked on x86, works on ARM Linux (with GCC) as well. Not sure if it has
anything to do with long doubles being 64bit there, but I don't think it
should.

src/Magnum/Math/Test/PackingTest.cpp

@@ -215,6 +215,9 @@ void PackingTest::packSigned() {
     {
         #if defined(CORRADE_TARGET_MSVC) || (defined(CORRADE_TARGET_ANDROID) && defined(CORRADE_TARGET_32BIT) && defined(CORRADE_TARGET_X86))
         CORRADE_EXPECT_FAIL("Long double (de)normalization is broken on MSVC and 32-bit x86 Android.");
+        #elif defined(CORRADE_TARGET_APPLE) && defined(CORRADE_TARGET_ARM)
+        /* Returns std::numeric_limits<Long>::min() */
+        CORRADE_EXPECT_FAIL("Negative signed long double (de)normalization is broken on ARM Mac.");
         #endif
         CORRADE_COMPARE(Math::pack<Long>(-1.0l), std::numeric_limits<Long>::min()+1);
     }