x86: tls.c declare sys_set_thread_area and sys_get_thread_area before they get used

Impact: cleanup

In asm/syscalls.h move out sys_set_thread_area() and sys_get_thread_area()
as they are common for both 32 and 64 bit.

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