sandbox: errno: avoid conflict with libc's errno

When building with LTO, the system libc's `errno` variable used in
arch/sandbox/cpu/os.c conflicts with U-Boot's `errno` (defined in
lib/errno.c) with the following error:
 .../ld: errno@@GLIBC_PRIVATE: TLS definition in /lib64/libc.so.6
         section .tbss mismatches non-TLS reference in
	 /tmp/u-boot.EQlEXz.ltrans0.ltrans.o

To avoid this conflict use different asm label for this variable when
CONFIG_SANDBOX is enabled.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/errno.h

@@ -8,7 +8,13 @@
 
 #include <linux/errno.h>
 
-extern int errno;
+#ifdef __SANDBOX__
+#define __errno_asm_label asm("__u_boot_errno")
+#else
+#define __errno_asm_label
+#endif
+
+extern int errno __errno_asm_label;
 
 #define __set_errno(val) do { errno = val; } while (0)
 

lib/errno.c

@@ -1 +1,3 @@
-int errno = 0;
+#include <errno.h>
+
+int errno __errno_asm_label = 0;