x86, vdso: Move syscall and sysenter setup into kernel/cpu/common.c

This code is used during CPU setup, and it isn't strictly speaking
related to the 32-bit vdso.  It's easier to understand how this
works when the code is closer to its callers.

This also lets syscall32_cpu_init be static, which might save some
trivial amount of kernel text.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/4e466987204e232d7b55a53ff6b9739f12237461.1399317206.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>