Bootstrap: Check validity of --target and --host triples before starting a build

Resolves rust-lang#122128

Author: Rajveer100

src/bootstrap/src/core/sanity.rs

@@ -177,6 +177,41 @@ than building it.
             continue;
         }
 
+        // Check if there exists a built-in target in the list of supported targets.
+        let cur_dir = env::current_dir().unwrap().display().to_string();
+        
+        let supported_target_file = format!("{cur_dir}/compiler/rustc_target/src/spec/mod.rs");
+        let contents = fs::read_to_string(supported_target_file)
+            .expect("Should have been able to read the file.");
+
+        let (mut macro_beg, mut macro_end) = (false, false);
+        let mut has_target = false;
+
+        let target_str = target.to_string();
+        for line in contents.lines() {
+            if macro_end { 
+                break; 
+            }
+            if !macro_beg && line.contains("supported_targets!") {
+                macro_beg = true;
+            } else if macro_beg {
+                if line.contains("}") {
+                    macro_end = true;
+                } else if line.contains(&target_str) {
+                    has_target = true;
+                }
+            }
+        }
+
+        // If not, check for a valid file location that may have been specified 
+        // by the user for the custom target.
+        // todo!()
+
+        if !has_target {
+            panic!("No such target exists in the target list, 
+                specify a correct location of the JSON specification file for custom targets!")
+        }
+
         if !build.config.dry_run() {
             cmd_finder.must_have(build.cc(*target));
             if let Some(ar) = build.ar(*target) {