libkern: tidy up memset
- Remove special-casing of 0 as it just results in an extra function call.
This is clearly pessimal.
- Drop the inline stuff. For the most part it is much better served with
__builtin_memset (coming later).
- Move the declaration to systm.h to match other funcs.
Archs are encouraged to implement the variant for their own platform so that
this implementation can be dropped.