Ensure __set_errno is still visible on LP32.

The use of the .hidden directive to avoid going via the PLT for
__set_errno had the side-effect of actually making __set_errno
hidden (which is odd because assembler directives don't usually
affect symbols defined in a different file --- you can't even
create a weak reference to a symbol that's defined in a different
file).

This change switches the system call stubs over to a new always-hidden
__set_errno_internal and has a visible __set_errno on LP32 just for
binary compatibility with old NDK apps.

Bug: 17423135
Change-Id: I6b6d7a05dda85f923d22e5ffd169a91e23499b7b
diff --git a/libc/arch-arm64/syscalls/mlockall.S b/libc/arch-arm64/syscalls/mlockall.S
index 3602d71..d4ca185 100644
--- a/libc/arch-arm64/syscalls/mlockall.S
+++ b/libc/arch-arm64/syscalls/mlockall.S
@@ -2,15 +2,13 @@
 
 #include <private/bionic_asm.h>
 
-    .hidden __set_errno
-
 ENTRY(mlockall)
     mov     x8, __NR_mlockall
     svc     #0
 
     cmn     x0, #(MAX_ERRNO + 1)
     cneg    x0, x0, hi
-    b.hi    __set_errno
+    b.hi    __set_errno_internal
 
     ret
 END(mlockall)