Add RLIMIT_* constants for RISC-V 64-bit GNU/Linux and minor fixes for type definitions

Fix #1670 missing RLIMIT_AS etc on android

Signed-off-by: Robert Collins <robertc@robertcollins.net>

Update Ubuntu images to 20.04

Add a workaround for disk space failures on Docker

Add CPU_ALLOC_SIZE

Co-Authored-By: Yuki Okushi <huyuumi.dev@gmail.com>

freebsdlike: add SCM_CREDS, struct sockcred

Like on NetBSD, but without a pid field in the struct.

It is present on both FreeBSD and DragonFly

Add CPU_COUNT/CPU_COUNT_S

revenge of #598

Add `rtentry` to musl

REG_ENOSYS is NetBSD specific

Set rustup profile to minimal on Windows

Minor clean-up CI docs

Bump version to 0.2.69

Enable `wasm32-unknown-emscripten` target on CI