Move errno to a pthread_internal_t field.

This change is intended to allow native-bridge to use independent
TLS memory for host and guest environments, while still sharing a
thread-local errno between the two.

Bug: http://b/78026329
Test: bionic unit tests
Change-Id: I838cd321e159add60760bc12a8aa7e9ddc960c33
Merged-In: I838cd321e159add60760bc12a8aa7e9ddc960c33
(cherry picked from commit a9c7c55462)

libc/bionic/__errno.cpp

@@ -29,8 +29,8 @@
 #include <errno.h>
 #include <stdint.h>
 
-#include "private/bionic_tls.h"
+#include "pthread_internal.h"
 
 int*  __errno() {
-  return reinterpret_cast<int*>(&(__get_tls()[TLS_SLOT_ERRNO]));
+  return &__get_thread()->errno_value;
 }

libc/bionic/pthread_internal.h

@@ -145,6 +145,8 @@ class pthread_internal_t {
 
   bionic_tls* bionic_tls;
 
+  int errno_value;
+
   // The thread pointer (__get_tls()) points at this field. This field must come last so that
   // an executable's TLS segment can be allocated at a fixed offset after the thread pointer.
   void* tls[BIONIC_TLS_SLOTS];

libc/private/bionic_tls.h

@@ -56,7 +56,8 @@ __BEGIN_DECLS
 enum {
   TLS_SLOT_SELF = 0, // The kernel requires this specific slot for x86.
   TLS_SLOT_THREAD_ID,
-  TLS_SLOT_ERRNO,
+
+  // TLS slot 2 was used for errno but is now free.
 
   // These two aren't used by bionic itself, but allow the graphics code to
   // access TLS directly rather than using the pthread API.