[MC] Leverage constexpr `std::array` in `SubtargetFeature.h`

Replace C-style array with `std::array` since `std::array<T, N>::operator[]` is
`constexpr` in C++17. This also allows us to replace `array_lengthof` calls with
member `size()` function.

Differential Revision: https://reviews.llvm.org/D131826