capsicum: use table for syscall policing

Using a static table is ~30% faster that working through
a switch statement.
capsicum: add fget rights to lustre driver
capsicum: fix compiler warnings
capsicum: tidy conditional compilation

 - Capsicum relies on CONFIG_SECCOMP
 - The capsicum_* fallback LSM hooks need to be defined even when
   CONFIG_SECURITY_CAPSICUM is not.
capsicum: make syscall checks less arch-specific

arch_prctl is x86-specific, so mark is as such.

Also remove the explicit check for ARCH_X86_64.