Add probe_sysroot_crate and emit_sysroot_crate

[leo60228]

See indexmap-rs/indexmap#95 (comment).

[cuviper]

Thanks! Just a few minor tweaks, please.

[cuviper]

Thanks again -- I've published autocfg 0.1.6.

[gnzlbg]

I suspect this (the 0.1.6 release) causes a segfault that breaks libc crate CI on x86_64-unknown-freebsd: https://cirrus-ci.com/task/6196089037783040

[cuviper]

I suspect this (the 0.1.6 release) causes a segfault

That would be really weird, since there's no unsafe, and this PR should just be dead code if you're not using it. But it seems you've now identified a different root cause in rust-lang/libc#1489.