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
diff --git a/libc/arch-mips64/syscalls/__exit.S b/libc/arch-mips64/syscalls/__exit.S
index ebee0d1..2d5e03d 100644
--- a/libc/arch-mips64/syscalls/__exit.S
+++ b/libc/arch-mips64/syscalls/__exit.S
@@ -2,8 +2,6 @@
 
 #include <private/bionic_asm.h>
 
-    .hidden __set_errno
-
 ENTRY(__exit)
     .set push
     .set noreorder
@@ -19,7 +17,7 @@
     nop
 2:
     .cpsetup ra, t1, 2b
-    LA t9,__set_errno
+    LA t9,__set_errno_internal
     .cpreturn
     j t9
     move ra, t0