Remove obsolete flag for WINARM64, fix TBB library lookup on Windows by andrjohns · Pull Request #241 · RcppCore/RcppParallel

Now that RcppParallel is using a newer version of the TBB, the TBB_USE_GCC_BUILTINS flag is irrelevant on Windows arm64

The tbbLibraryPath() function was also looking for .dll files on windows, so wasn't returning the static lib