[libc] memmove optimizations (#70043)

1. Remove is_disjoint check for smaller sizes and reduce code bloat.

inline_memmove may handle some small sizes as efficiently
as inline_memcpy. For these sizes we may not do is_disjoint check.
This both avoids additional code for the most frequent smaller sizes
and removes code bloat (we don't need the memcpy logic for small sizes).
Here we heavily rely on inlining and dead code elimination: from the
first
inline_memmove we should get only handling of small sizes, and from
the second inline_memmove and inline_memcpy we should get only handling
of larger sizes.

2. Use the memcpy thresholds for memmove.
Memcpy thresholds were more carefully tuned.
This becomes more important since we use memmove
for all small sizes always now.

3. Fix boundary conditions for sizes = 16/32/64.
See the added comment for explanations.

Memmove function size drops from 885 to 715 bytes
due to removed duplication.

```
                 │  baseline   │          ...