Rollup merge of #47352 - EdSchouten:cloudabi-libtest, r=kennytm
Rollup merge of #47352 - EdSchouten:cloudabi-libtest, r=kennytm

Make libtest build on CloudABI.

Just like on UNIX systems, we need to use sysconf() to obtain the number
of CPUs. Extend the existing cfg()'s to match CloudABI as well.

GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust