buildroot

ljsyscall: remove dependency on some architecture

ljsyscall is available for all arch supported by LuaJIT.
refactor LJSYSCALL_ARCH : dispatch all arch.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>