Hide the __bionic_clone and __bionic_clone_entry implementation details.

clone(2) is the public symbol.

Also switch a test from __bionic_clone to clone; testing public API
means the test now works on glibc too.

Change-Id: If59def26a00c3afadb8a6cf9442094c35a59ffde
diff --git a/libc/bionic/clone.cpp b/libc/bionic/clone.cpp
index 1d997fe..4ea3c70 100644
--- a/libc/bionic/clone.cpp
+++ b/libc/bionic/clone.cpp
@@ -35,7 +35,7 @@
 extern "C" void __exit(int status);
 
 // Called from the __bionic_clone assembler to call the thread function then exit.
-extern "C" void __bionic_clone_entry(int (*fn)(void*), void* arg) {
+extern "C" __LIBC_HIDDEN__ void __bionic_clone_entry(int (*fn)(void*), void* arg) {
   int status = (*fn)(arg);
   __exit(status);
 }