Make __clone_for_fork pure native bridge mode abstraction

In native build of libc it would be inlined and in native bridge mode
it's noinline, extern "C" and thus could be easily intercepted.

Test: m (without weak symbols in native bridge mode x86+arm build would be broken)

Change-Id: I67759858a5bc2174dce1db9732fdbd89ba7689cc
diff --git a/libc/private/bionic_defs.h b/libc/private/bionic_defs.h
index c0ae1b0..5a48f25 100644
--- a/libc/private/bionic_defs.h
+++ b/libc/private/bionic_defs.h
@@ -36,9 +36,12 @@
 #ifdef __ANDROID_NATIVE_BRIDGE__
 #define __BIONIC_WEAK_FOR_NATIVE_BRIDGE __attribute__((__weak__, __noinline__))
 #define __BIONIC_WEAK_VARIABLE_FOR_NATIVE_BRIDGE __attribute__((__weak__))
+#define __BIONIC_WEAK_FOR_NATIVE_BRIDGE_INLINE \
+  __BIONIC_WEAK_FOR_NATIVE_BRIDGE extern "C" __LIBC_HIDDEN__
 #else
 #define __BIONIC_WEAK_FOR_NATIVE_BRIDGE
 #define __BIONIC_WEAK_VARIABLE_FOR_NATIVE_BRIDGE
+#define __BIONIC_WEAK_FOR_NATIVE_BRIDGE_INLINE static inline
 #endif
 
 #endif /* __BIONIC_PRIVATE_BIONIC_DEFS_H_ */