[libc++] Provide aligned allocation function iff the compiler claims support (#219199)

There isn't much of a point to providing declarations for aligned
allocation functions but not allowing the compiler to use them. So
either aligned allocations should be allowed to be used via
`-faligned-allocation` or not provided at all.

GitOrigin-RevId: e8d2def7e7ed18f22a493b72435468225c1047ba