x86: even faster core_id()s with segmentation

The old slow core_id reads, using the LAPIC and the os_coreid() lookup
took about 50 TSC ticks per core_id() call (avg time in a for loop of
100000 calls).

The rdtscp option (the old FAST_COREID) was about 30 ticks.

The segmentation lookup is 1 tick.

rdtscp still has the pcoreid in ecx, so that userspace can use it.