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-x86/bionic/__bionic_clone.S b/libc/arch-x86/bionic/__bionic_clone.S
index 917dc68..ef78aee 100644
--- a/libc/arch-x86/bionic/__bionic_clone.S
+++ b/libc/arch-x86/bionic/__bionic_clone.S
@@ -32,7 +32,7 @@
         # An error occurred, so set errno and return -1.
         negl    %eax
         pushl   %eax
-        call    __set_errno
+        call    __set_errno_internal
         addl    $4, %esp
         jmp     .L_bc_return
 
diff --git a/libc/arch-x86/bionic/syscall.S b/libc/arch-x86/bionic/syscall.S
index 8e76c4e..f85ec39 100644
--- a/libc/arch-x86/bionic/syscall.S
+++ b/libc/arch-x86/bionic/syscall.S
@@ -38,7 +38,7 @@
     # Yes, so set errno.
     negl    %eax
     pushl   %eax
-    call    __set_errno
+    call    __set_errno_internal
     addl    $4, %esp
 1:
     # Restore the callee save registers.
diff --git a/libc/arch-x86/bionic/vfork.S b/libc/arch-x86/bionic/vfork.S
index ffa6b16..6c02910 100644
--- a/libc/arch-x86/bionic/vfork.S
+++ b/libc/arch-x86/bionic/vfork.S
@@ -38,7 +38,7 @@
   jb      1f
   negl    %eax
   pushl   %eax
-  call    __set_errno
+  call    __set_errno_internal
 1:
   jmp     *%ecx  // Jump to the stored return address.
 END(vfork)