common-user: Move safe-syscall.* from linux-user

Move linux-user safe-syscall.S and safe-syscall-error.c to common-user
so that bsd-user can also use it.  Also move safe-syscall.h to
include/user/.  Since there is nothing here that is related to the guest,
as opposed to the host, build it once.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/signal.c

@@ -31,7 +31,7 @@
 #include "trace.h"
 #include "signal-common.h"
 #include "host-signal.h"
-#include "safe-syscall.h"
+#include "user/safe-syscall.h"
 
 static struct target_sigaction sigact_table[TARGET_NSIG];