Auto merge of #124798 - devnexen:illumos_memalign_fix, r=RalfJung

std::alloc: use posix_memalign instead of memalign on solarish

`memalign` on Solarish requires the alignment to be at least the size of a pointer, which we did not honor. `posix_memalign` also requires that, but that code path already takes care of this requirement.

close GH-124787

Author: bors