Simplify check condition for memcpy path in memmove

[MaxGraey]

Ref: https://git.musl-libc.org/cgit/musl/tree/src/string/memmove.c

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[dcodeIO]

Relevant musl commit for background:

the comparison must take place in the address space model as an
integer type, since comparing pointers that are not pointing into the
same array is undefined.

the subsequent d<s comparison however is valid, because it's only
reached in the case where the source and dest overlap, in which case
they are necessarily pointing to parts of the same array.

to make the comparison, use an unsigned range check for dist(s,d)>=n,
algebraically !(-n<s-d<n). subtracting n yields !(-2n<s-d-n<0), which
mapped into unsigned modular arithmetic is !(-2n<s-d-n) or rather
-2*n>=s-d-n.

[github-actions]

🎉 This PR is included in version 0.14.7 🎉

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