musl: avoid multiple evaluations in CPU_ISSET_S() macro

musl: avoid multiple evaluations in CPU_ISSET_S() macro

musl's CPU_ISSET_S() macro does not avoid multiple evaluations, and it
only accepts simple variable or constant.

Fixes the following error.
```
../src/shared/cpu-set-util.c: In function ‘cpu_set_to_mask_string’:
../src/shared/cpu-set-util.c:101:41: warning: operation on ‘i’ may be undefined [-Werror=sequence-point]
  101 |                         if (CPU_ISSET_S(--i, c->allocated, c->set))
      |                                         ^
```