Disable vptr checks for Singleton constructor

[oold]

UBSan complains a lot about the this pointer not being of type T* which is correct during construction, but ultimately doesn't matter. This change disables the vptr check on the Singleton constructor to avoid errors from UBSan.

[paroj]

oh the joy of cross-platform code 😉

The docs say you cant check for __has_attribute like that:
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fattribute.html

which means that you can use the recommended version, and the GNUC check is redundant.

[oold]

Oh, wow. I even tested this with different versions of GCC and Clang.
Also very interesting: __has_attribute doesn't seem to be documented until GCC 9, but even GCC 5 seems to know it.
I was actually quite sure that the way I did it wouldn't work, so I was surprised when the old GCCs accepted the code.