Hide the symbol of helper function __libc_android_abort

Ideally __libc_android_abort would be static, but it could not be
because gcc would not allow calling a static function from an asm
statement.  Instead, using GCC visibility is work around.

Change-Id: Ifff6b9957ca3f0fc03c75c3e42582a48d43cefa2

libc/unistd/abort.c

@@ -40,7 +40,7 @@
     __libc_android_log_print(ANDROID_LOG_DEBUG, "libc-abort", (format), ##__VA_ARGS__ )
 
 #ifdef __arm__
-void
+__LIBC_HIDDEN__ void
 __libc_android_abort(void)
 #else
 void