Make __set_errno hidden in asm.

This fixes the build after the -Bsymbolic change.

Bug: 16853291
Change-Id: I989c9fec3c32e0289ea257a3bd2b7fd2709b6ce2
diff --git a/libc/arch-arm/syscalls/fdatasync.S b/libc/arch-arm/syscalls/fdatasync.S
index 9c8317d..9556665 100644
--- a/libc/arch-arm/syscalls/fdatasync.S
+++ b/libc/arch-arm/syscalls/fdatasync.S
@@ -2,6 +2,8 @@
 
 #include <private/bionic_asm.h>
 
+    .hidden __set_errno
+
 ENTRY(fdatasync)
     mov     ip, r7
     ldr     r7, =__NR_fdatasync