Unix: add aligned_alloc

[devnexen]

close #3689

[rustbot]

r? @tgross35

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[tgross35]

Great, thanks!

[tgross35]

Source links, copied from #3698:

• linux glibc https://github.com/bminor/glibc/blob/b22923abb046311ac9097a36b97b9b97342bac44/stdlib/stdlib.h#L724
• linux musl https://github.com/kraj/musl/blob/2c41ee96f60074fa8ac387e1fcdedba0654fb173/include/stdlib.h#L44
• linux ulibc https://github.com/wbx-github/uclibc-ng/blob/ecbc412d0d93c8b7b77bec6539b290c9da5401cf/include/stdlib.h#L536
• newlib https://github.com/reswitched/newlib/blob/76f6d4e21ed5b621adb75fe99a69704dca831177/newlib/libc/stdlib/aligned_alloc.c#L30
• openbsd https://github.com/openbsd/src/blob/fd042ffdc70d1726d6308c4819514edac16eff53/lib/libc/stdlib/malloc.c#L2342-L2343
• netbsd https://man.netbsd.org/aligned_alloc.3
• freebsd https://man.freebsd.org/cgi/man.cgi?query=aligned_alloc&apropos=0&sektion=3&manpath=FreeBSD+9-current&format=html
• redox https://github.com/redox-os/relibc/blob/a38638542d0f7b0063da3c84833bd72c0f71da88/src/header/stdlib/mod.rs#L122
• haiku https://github.com/haiku/haiku/blob/ba4478ce60f4c0c3f00697981fc87d4e9e66b230/headers/posix/stdlib.h#L52
• aix (well z/os, but I assume they would be the same) https://www.ibm.com/docs/en/zos/2.5.0?topic=functions-aligned-alloc-allocating-aligned-memory-blocks
• nto/qnx https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino.lib_ref/topic/a/aligned_alloc.html
• Apple: unknown, but seems to be available in recent OS versions