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/bionic/__bionic_clone.S b/libc/arch-x86/bionic/__bionic_clone.S
index e6ddaaa..bc7f71d 100644
--- a/libc/arch-x86/bionic/__bionic_clone.S
+++ b/libc/arch-x86/bionic/__bionic_clone.S
@@ -51,3 +51,4 @@
         popl    %ebx
         ret
 END(__bionic_clone)
+.hidden __bionic_clone