linux-user: Define TARGET_ERESTART* errno values

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: Timothy 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: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>