[PAC][libunwind] Refactor way of describing internal signing schemas

[kovdan01]

#143230 introduces many macros for internal signing schemas. Let's adopt @atrosinenko 's approach to make the code more readable. See original comment from the PR below.

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(As an illustration for the "cover letter" of this review) I assume many such macroses define purely internal signing schemas. In such cases it should be possible to compute the discriminator automatically - it could probably improve readability to use something like

    PTRAUTH_INTPTR(uintptr_t, start) = readEncodedPointer(&p, callSiteEncoding);

instead of

    uintptr_t __ptrauth_gcc_personality_start start =
        readEncodedPointer(&p, callSiteEncoding);

Originally posted by @atrosinenko in #143230 (comment)

[kovdan01]

Tagging @asl @ojhunt @atrosinenko

[ojhunt]

I'm not sure this is a readability reason, and automated discriminators means qualifiers that have to be re-used have to be written differently.