Make __set_errno hidden in asm.

This fixes the build after the -Bsymbolic change.

Bug: 16853291
Change-Id: I989c9fec3c32e0289ea257a3bd2b7fd2709b6ce2
diff --git a/libc/tools/gensyscalls.py b/libc/tools/gensyscalls.py
index 42f2c91..4619ec6 100755
--- a/libc/tools/gensyscalls.py
+++ b/libc/tools/gensyscalls.py
@@ -48,6 +48,8 @@
 """
 #include <private/bionic_asm.h>
 
+    .hidden __set_errno
+
 ENTRY(%(func)s)
 """