cmList: Use ptrdiff_t for index_type

In commit 87fe031a07 (cmList class: various enhancements, 2023-04-26,
v3.27.0-rc1~142^2~1) this changed from `int` to `intptr_t`, but on some
systems (e.g. CHERI-enabled Arm or RISC-V), `intptr_t` is larger than 64
bits and using it for `index_type` here incurs an unnecessary
performance penalty.  Additionally, using `intptr_t` here results in an
ambiguous overload while calling `cmStrCat` with `intptr_t`: `error:
conversion from '__intcap' to 'const cmAlphaNum' is ambiguous`.

Instead of adding `intptr_t` overloads for `cmAlphaNum` for CHERI
systems, we can change the type to `ptrdiff_t` (which will be the same
as `intptr_t` on all other systems) and avoid carrying diffs that only
compile with a CHERI enabled compiler.

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