Remove volatile from the __errno declaration.

This breaks code that declares `errno` itself for whatever crazy reason:

  b.c:22:12: error: conflicting types for '__errno'
  extern int errno;
             ^
  /usr/local/google/ndkports/toolchain/bin/../sysroot/usr/include/errno.h:47:20: note: expanded from macro 'errno'
  #define  errno   (*__errno())
                     ^
  /usr/local/google/ndkports/toolchain/bin/../sysroot/usr/include/errno.h:44:15: note: previous declaration is here
  volatile int* __errno(void) __attribute_const__;
                ^

Bug: N/A
Test: built various bits of GNU source
Change-Id: I27c03bf3bde419a001f98f1ea6c267c847f31271

libc/bionic/__errno.cpp

@@ -31,6 +31,6 @@
 
 #include "private/bionic_tls.h"
 
-volatile int*  __errno() {
+int*  __errno() {
   return reinterpret_cast<int*>(&(__get_tls()[TLS_SLOT_ERRNO]));
 }

libc/include/errno.h

@@ -25,6 +25,7 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _ERRNO_H
 #define _ERRNO_H
 
@@ -33,18 +34,13 @@
 
 __BEGIN_DECLS
 
-/* on Linux, ENOTSUP and EOPNOTSUPP are defined as the same error code
- * even if 1000.3 states that they should be different
- */
-#ifndef  ENOTSUP
-#define  ENOTSUP  EOPNOTSUPP
+/* On Linux, ENOTSUP and EOPNOTSUPP are the same despite POSIX saying they should be distinct. */
+#ifndef ENOTSUP
+#define ENOTSUP EOPNOTSUPP
 #endif
 
-/* internal function returning the address of the thread-specific errno */
-volatile int* __errno(void) __attribute_const__;
-
-/* a macro expanding to the errno l-value */
-#define  errno   (*__errno())
+int* __errno(void) __attribute_const__;
+#define errno (*__errno())
 
 __END_DECLS
 

tests/Android.bp

@@ -61,6 +61,7 @@ cc_test_library {
         "dirent_test.cpp",
         "elf_test.cpp",
         "endian_test.cpp",
+        "errno_test.cpp",
         "error_test.cpp",
         "eventfd_test.cpp",
         "fcntl_test.cpp",

tests/errno_test.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <gtest/gtest.h>
+
+#include <errno.h>
+
+// Some GNU source likes to declare errno itself for some reason.
+extern "C" int errno;