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/arch-x86_64/bionic/__bionic_clone.S b/libc/arch-x86_64/bionic/__bionic_clone.S
index cf98d76..c2ebebd 100644
--- a/libc/arch-x86_64/bionic/__bionic_clone.S
+++ b/libc/arch-x86_64/bionic/__bionic_clone.S
@@ -70,3 +70,4 @@
 2:
         ret
 END(__bionic_clone)
+.hidden __bionic_clone