Reimplement the lazy FP context switching:

Reimplement the lazy FP context switching:
o   Move all code into a single file for easier maintenance.
o   Use a single global lock to avoid having to handle either
    multiple locks or race conditions.
o   Make sure to disable the high FP registers after saving
    or dropping them.
o   use msleep() to wait for the other CPU to save the high
    FP registers.

This change fixes the high FP inconsistency panics.

A single global lock typically serializes too much, which may
be noticable when a lot of threads use the high FP registers,
but in that case it's probably better to switch the high FP
context synchronuously. Put differently: cpu_switch() should
switch the high FP registers if the incoming and outgoing
threads both use the high FP registers.