Modern C runtimes have well optimized memset and memcpy, so use those instead of dispatching into SDL's versions. In addition, some compilers can analyze memset and memcpy calls and directly turn them into optimized assembly.
This is done such that we can disable LTO for these 2 functions when
building with MSVC.
This is due to a limitation of Link Time Code Generation (LTCG).
Code generation might generate a new reference to memset after linking
has started. The LTCG must make assumptions about where memset is
defined which is normally the C runtime.