Altivec: fix compilation with C++20 and higher

What does this implement/fix?

This fixes compilation of the Altivec code with -std=c++20 and -std=c++23. Use of a simple-template-id as name of the constructor seems to have been allowed in C++17 and earlier, but does not really add anything. The simple class name works with -std=c++98 as well.

Additional information

Problem discovered when compiling tflite as a dependency of chromium, which uses -std=gnu++2a.