x86: process_64.c declare __switch_to() and sys_arch_prctl before they get used

Impact: cleanup

In asm/system.h moved out __switch_to from CONFIG_X86_32 as it is common for
both 32 and 64 bit.

In asm/pctl.h defined sys_arch_prctl
Signed-off-by: Jaswinder Singh <jaswinder@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>