Reading and writing to an sa_restorer member that isn't supposed to
exist corrupts user memory.  Introduce TARGET_ARCH_HAS_SA_RESTORER,
similar to the kernel's __ARCH_HAS_SA_RESTORER.
Reported-by: NHelge Deller <deller@gmx.de>
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>