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.

(cherry-pick of 7efad83d430f4d824f2aaa75edea5106f6ff8aae.)

Bug: 17423135
Change-Id: I6b6d7a05dda85f923d22e5ffd169a91e23499b7b

libc/arch-arm/syscalls/sigaltstack.S

@@ -2,8 +2,6 @@
 
 #include <private/bionic_asm.h>
 
-    .hidden __set_errno
-
 ENTRY(sigaltstack)
     mov     ip, r7
     ldr     r7, =__NR_sigaltstack
@@ -12,5 +10,5 @@ ENTRY(sigaltstack)
     cmn     r0, #(MAX_ERRNO + 1)
     bxls    lr
     neg     r0, r0
-    b       __set_errno
+    b       __set_errno_internal
 END(sigaltstack)