posix_spawn: fix for some custom allocator setups

libc cannot assume that aligned_alloc and free come from jemalloc, or that
any application providing its own malloc and free is actually providing
aligned_alloc.

Switch back to malloc and just make sure we're passing a properly aligned
stack into rfork_thread, as an application perhaps can't reasonably replace
just malloc or just free without headaches.

This unbreaks ksh93 after r361996, which provides malloc/free but no
aligned_alloc.

Reported by: freqlabs
Diagnosed by: Andrew Gierth <andrew_tao173.riddles.org.uk>
X-MFC-With: r361996