[libc] Add a set of elementary operations
Resubmission of D100646 now making sure that we handle cases were __builtin_memcpy_inline is not available.
Original commit message:
Each of these elementary operations can be assembled to support higher order constructs (Overlapping access, Loop, Aligned Loop).
The patch does not compile yet as it depends on other ones (D100571, D100631) but it allows to get the conversation started.
A self-contained version of this code is available at https://godbolt.org/z/e1x6xdaxM