rustc: Don't export builtins/panic/alloc syms

This hides symbols from various unstable and implementation-detail
crates of the standard library. Although typically transitive exported
`pub extern` functions are exported from cdylibs, these crates aren't
necessary as they're all implementation details.

Closes #34493

src/liballoc_jemalloc/lib.rs

@@ -143,7 +143,7 @@ mod imp {
     // we're building on doesn't have them defined, so just make sure the symbols
     // are available.
     #[no_mangle]
-    #[cfg(target_os = "android")]
+    #[cfg(all(target_os = "android", not(cargobuild)))]
     pub extern "C" fn pthread_atfork(_prefork: *mut u8,
                                      _postfork_parent: *mut u8,
                                      _postfork_child: *mut u8)