When an architecture is unsupported arch.h would silently continue.
This leads to a lot of followup errors because my_syscallX() is not
defined and the startup code is missing.

Avoid these confusing errors and fail the build early with a clear
error message and location.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>