moz_posix_memalign is a wrapper for posix_memalign that only exists if
posix_memalign exists.
On OSX, it has a fallback for an under-specified bug where it
purportedly returns a pointer that doesn't have the requested alignment.
That fallback was added in bug 414946, over 6 years ago, before jemalloc
was even enabled on OSX.
Considering posix_memalign is used directly in many other places in
Gecko, that we almost always use mozjemalloc, which doesn't have these
problems, and that in all likeliness, the bug was in some old version of
OSX that is not supported anymore, the fallback does not seem all that
useful.
So, just use posix_memalign directly.
Ehsan Akhgari