Illumos: make basic std facilities work

[RalfJung]

We're currently lacking some basics on Illumos:

• high-alignment heap allocations (MIRI_TEST_TARGET=x86_64-unknown-illumos ./miri test alloc): needs support for libc::memalign.
• HashMap random initialization (MIRI_TEST_TARGET=x86_64-unknown-illumos ./miri test hashmap): this complains about open64 in isolation; the issue is that std falls back to file-based randomness, but we want it to use one of the native randomness APIs. Apparently this will need a patch for the standard library; currently it has no support for getrandom on Solarish. Solarish has libc::getrandom so it should be trivial to extend this to also cover these OSes.

Cc @devnexen

This issue has been assigned to @devnexen via this comment.

[RalfJung]

@rustbot assign @devnexen
you seem to be working on this?