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-arm/bionic/__bionic_clone.S b/libc/arch-arm/bionic/__bionic_clone.S
index 7b76f5e..7cc4db5 100644
--- a/libc/arch-arm/bionic/__bionic_clone.S
+++ b/libc/arch-arm/bionic/__bionic_clone.S
@@ -71,3 +71,4 @@
     ldr    r1, [sp, #-8]
     b      __bionic_clone_entry
 END(__bionic_clone)
+.hidden __bionic_clone