arch/x86: (Intel64) use TSS for per-CPU variables

A space is allocated in the TSS for per-CPU variables. At present,
this is only a 'struct _cpu *' to find the _kernel CPU struct. The
locore routines are rewritten to find _current and _nested via this
pointer rather than referencing the _kernel global directly.

This is obviously in preparation for SMP support.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>