Add options for enabling overflow checks in rustc and std.

The options are `overflow-checks` and `overflow-checks-std`
defaulting to false.

src/bootstrap/builder.rs

@@ -1205,6 +1205,14 @@ impl<'a> Builder<'a> {
                 self.config.rust_debug_assertions.to_string()
             },
         );
+        cargo.env(
+            profile_var("OVERFLOW_CHECKS"),
+            if mode == Mode::Std {
+                self.config.rust_overflow_checks_std.to_string()
+            } else {
+                self.config.rust_overflow_checks.to_string()
+            },
+        );
 
         // `dsymutil` adds time to builds on Apple platforms for no clear benefit, and also makes
         // it more difficult for debuggers to find debug info. The compiler currently defaults to