Avoid blocking the main loop when editing Cargo.toml

I've noticed a bunch of "main loop too long" warnings in console when
typing in Cargo.toml. Profiling showed that the culprit is `rustc
--print cfg` call.

I moved it to the background project loading phase, where it belongs.
This highlighted a problem: we generally use single `cfg`, while it
really should be per crate.

crates/project_model/src/rustc_cfg.rs

@@ -0,0 +1,34 @@
+//! Runs `rustc --print cfg` to get built-in cfg flags.
+
+use std::process::Command;
+
+use crate::{cfg_flag::CfgFlag, utf8_stdout};
+
+pub(crate) fn get(target: Option<&str>) -> Vec<CfgFlag> {
+    let _p = profile::span("rustc_cfg::get");
+    let mut res = Vec::new();
+
+    // Some nightly-only cfgs, which are required for stdlib
+    res.push(CfgFlag::Atom("target_thread_local".into()));
+    for &ty in ["8", "16", "32", "64", "cas", "ptr"].iter() {
+        for &key in ["target_has_atomic", "target_has_atomic_load_store"].iter() {
+            res.push(CfgFlag::KeyValue { key: key.to_string(), value: ty.into() });
+        }
+    }
+
+    let rustc_cfgs = {
+        let mut cmd = Command::new(toolchain::rustc());
+        cmd.args(&["--print", "cfg", "-O"]);
+        if let Some(target) = target {
+            cmd.args(&["--target", target]);
+        }
+        utf8_stdout(cmd)
+    };
+
+    match rustc_cfgs {
+        Ok(rustc_cfgs) => res.extend(rustc_cfgs.lines().map(|it| it.parse().unwrap())),
+        Err(e) => log::error!("failed to get rustc cfgs: {:#}", e),
+    }
+
+    res
+}