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/tools/gensyscalls.py b/libc/tools/gensyscalls.py
index 929bec4..4e24077 100755
--- a/libc/tools/gensyscalls.py
+++ b/libc/tools/gensyscalls.py
@@ -52,8 +52,6 @@
 """
 #include <private/bionic_asm.h>
 
-    .hidden __set_errno
-
 ENTRY(%(func)s)
 """
 
@@ -76,7 +74,7 @@
     cmn     r0, #(MAX_ERRNO + 1)
     bxls    lr
     neg     r0, r0
-    b       __set_errno
+    b       __set_errno_internal
 END(%(func)s)
 """
 
@@ -96,7 +94,7 @@
     cmn     r0, #(MAX_ERRNO + 1)
     bxls    lr
     neg     r0, r0
-    b       __set_errno
+    b       __set_errno_internal
 END(%(func)s)
 """
 
@@ -111,7 +109,7 @@
 
     cmn     x0, #(MAX_ERRNO + 1)
     cneg    x0, x0, hi
-    b.hi    __set_errno
+    b.hi    __set_errno_internal
 
     ret
 END(%(func)s)
@@ -132,7 +130,7 @@
     j ra
     nop
 1:
-    la t9,__set_errno
+    la t9,__set_errno_internal
     j t9
     nop
     .set reorder
@@ -159,7 +157,7 @@
     nop
 2:
     .cpsetup ra, t1, 2b
-    LA t9,__set_errno
+    LA t9,__set_errno_internal
     .cpreturn
     j t9
     move ra, t0
@@ -181,7 +179,7 @@
     jb      1f
     negl    %%eax
     pushl   %%eax
-    call    __set_errno
+    call    __set_errno_internal
     addl    $4, %%esp
 1:
 """
@@ -203,7 +201,7 @@
     jb      1f
     negl    %%eax
     movl    %%eax, %%edi
-    call    __set_errno
+    call    __set_errno_internal
 1:
     ret
 END(%(func)s)