str-graphemes: rework and add some zig tests

- rework strGraphemes() to use a mutable slice and keep track of just
`last_codepoint_len`.
- add zig tests for empty string, ascii, utf8, ascii+utf8+emoji

crates/compiler/build/src/link.rs

@@ -434,7 +434,12 @@ pub fn build_c_host_native(
             _ => {
                 command.args(&[
                     shared_lib_path.to_str().unwrap(),
-                    &bitcode::get_builtins_host_obj_path(),
+                    // This line is commented out because
+                    // @bhansconnect: With the addition of Str.graphemes, always
+                    // linking the built-ins led to a surgical linker bug for
+                    // optimized builds. Disabling until it is needed for dev
+                    // builds.
+                    // &bitcode::get_builtins_host_obj_path(),
                     "-fPIE",
                     "-pie",
                     "-lm",