Missing function: aligned_alloc

[RalfJung]

This function is part of the C standard. I don't know the full set of targets that support it.

• Linux: glibc has it, and I found it in musl, too (but that uses malloc + offset -- no idea how they're making sure that you can free the resulting pointer)
• FreeBSD also seems to have it
• macOS: available since 10.15 according to this (but buggy until today... but that's not libc's concern)
• Solaris seems to have it, and same for Illumos (again, buggy...)
• The function is mentioned here so I assume (recent enough) Android also has it.