PCU diffs for Alpha.

[Matt Thomas <matt%3am-software.com@localhost>]

http://www.netbsd.org/~matt/alpha-pcu-diffs.txt contains a set of diff against 
-HEAD moving alpha to use PCU for managing its FPU.  MIPS and PowerPC already 
use PCU.

One thing I did is stop leaving the FPU enabled after loading it.  If the 
kernel wants to use FPU, it should needs to enable it first.

I think I got everything right (GENERIC and GENERIC.MP both build) but I'd 
appreciate if someone could test the patch and let me know if it works.