linux-user: Define TARGET_ERESTART* errno values

Define TARGET_ERESTARTSYS; like the kernel, we will use this to
indicate that a guest system call should be restarted. We use
the same value the kernel does for this, 512.
Signed-off-by: NTimothy Edward Baldwin <T.E.Baldwin99@members.leeds.ac.uk>
[PMM: split out from the patch which moves and renumbers
 TARGET_QEMU_ESIGRETURN, add comment on usage]
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/errno_defs.h

@@ -139,3 +139,11 @@
 /* for robust mutexes */
 #define TARGET_EOWNERDEAD      130     /* Owner died */
 #define TARGET_ENOTRECOVERABLE 131     /* State not recoverable */
+
+/* QEMU internal, not visible to the guest. This is returned when a
+ * system call should be restarted, to tell the main loop that it
+ * should wind the guest PC backwards so it will re-execute the syscall
+ * after handling any pending signals. They match with the ones the guest
+ * kernel uses for the same purpose.
+ */
+#define TARGET_ERESTARTSYS     512     /* Restart system call (if SA_RESTART) */