Extract target no-std hack to build_helpers

To centralize this hack in one place with a backlink to the issue
tracking this hack, as this logic is also needed by compiletest to
implement a `//@ needs-target-std` directive.

Author: jieyouxu

src/bootstrap/src/core/config/toml/target.rs

@@ -84,7 +84,7 @@ pub struct Target {
 impl Target {
     pub fn from_triple(triple: &str) -> Self {
         let mut target: Self = Default::default();
-        if triple.contains("-none") || triple.contains("nvptx") || triple.contains("switch") {
+        if !build_helper::targets::target_supports_std(triple) {
             target.no_std = true;
         }
         if triple.contains("emscripten") {

src/build_helper/src/lib.rs

@@ -6,6 +6,7 @@ pub mod fs;
 pub mod git;
 pub mod metrics;
 pub mod stage0_parser;
+pub mod targets;
 pub mod util;
 
 /// The default set of crates for opt-dist to collect LLVM profiles.

src/build_helper/src/targets.rs

@@ -0,0 +1,11 @@
+// FIXME(#142296): this hack is because there is no reliable way (yet) to determine whether a given
+// target supports std. In the long-term, we should try to implement a way to *reliably* determine
+// target (std) metadata.
+//
+// NOTE: this is pulled out to `build_helpers` to share this hack between `bootstrap` and
+// `compiletest`.
+pub fn target_supports_std(target_tuple: &str) -> bool {
+    !(target_tuple.contains("-none")
+        || target_tuple.contains("nvptx")
+        || target_tuple.contains("switch"))
+}