Introduce memcpy_aligned<N>(), memcmp_aligned<N>(), memset_aligned<N>()
and use them for accessing InnoDB page header fields that are known
to be aligned.

MY_ASSUME_ALIGNED(): Wrapper for the GCC/clang __builtin_assume_aligned().
Nothing similar seems to exist in Microsoft Visual Studio, and the
C++20 std::assume_aligned is not available to us yet.

Explicitly specified alignment guarantees allow compilers to generate
faster code on platforms with strict alignment rules, instead of
emitting calls to potentially unaligned memcpy(), memcmp(), or memset().