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/include/sys/cdefs_elf.h

@@ -95,6 +95,10 @@
 	__asm__(".section _sec\n\t.asciz _str\n\t.previous")
 #endif
 
+/* GCC visibility helper macro */
+#define __LIBC_HIDDEN__							\
+	__attribute__ ((visibility ("hidden")))
+
 #define	__IDSTRING(_n,_s)		__SECTIONSTRING(.ident,_s)
 
 #define	__RCSID(_s)			__IDSTRING(rcsid,_s)