Re-enable some Rumprun tests

libc-test/build.rs

@@ -350,10 +350,6 @@ fn main() {
             // they're implemented on rumprun yet, just let them slide for now.
             // Some of them look like they have headers but then don't have
             // corresponding actual definitions either...
-            "backtrace" |
-            "pthread_main_np" |
-            "pthread_set_name_np" |
-            "pthread_stackseg_np" |
             "shm_open" |
             "shm_unlink" |
             "syscall" |