Allow getting no_std from the config file

Currently, it is only set correctly in the sanity checking implicit
default fallback code. Having a config file at all will for force
`no_std = false`.

src/bootstrap/config.rs

@@ -353,6 +353,7 @@ struct TomlTarget {
     musl_root: Option<String>,
     wasi_root: Option<String>,
     qemu_rootfs: Option<String>,
+    no_std: Option<bool>,
 }
 
 impl Config {
@@ -615,6 +616,8 @@ impl Config {
                 target.musl_root = cfg.musl_root.clone().map(PathBuf::from);
                 target.wasi_root = cfg.wasi_root.clone().map(PathBuf::from);
                 target.qemu_rootfs = cfg.qemu_rootfs.clone().map(PathBuf::from);
+                target.no_std =
+                    cfg.no_std.unwrap_or(triple.contains("-none-") || triple.contains("nvptx"));
 
                 config.target_config.insert(INTERNER.intern_string(triple.clone()), target);
             }

src/bootstrap/sanity.rs

@@ -194,9 +194,7 @@ pub fn check(build: &mut Build) {
 
         if target.contains("-none-") || target.contains("nvptx") {
             if build.no_std(*target).is_none() {
-                let target = build.config.target_config.entry(target.clone()).or_default();
-
-                target.no_std = true;
+                build.config.target_config.entry(target.clone()).or_default();
             }
 
             if build.no_std(*target) == Some(false) {