x86: clean up declarations and variables

Impact: cleanup, no code changed

 - syscalls.h       update declarations due to unifications
 - irq.c            declare smp_generic_interrupt() before it gets used
 - process.c        declare sys_fork() and sys_vfork() before they get used
 - tsc.c            rename tsc_khz shadowed variable
 - apic/probe_32.c  declare apic_default before it gets used
 - apic/nmi.c       prev_nmi_count should be unsigned
 - apic/io_apic.c   declare smp_irq_move_cleanup_interrupt() before it gets used
 - mm/init.c        declare direct_gbpages and free_initrd_mem before they get used

Signed-off-by: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>