MULTIPROCESSOR enabled by default on all Alpha kernels

[Jason Thorpe <thorpej%me.com@localhost>]

I just checked in a change that enables the MULTIPROCESSOR option by default in all NetBSD/alpha kernels.  Thanks to John Klos, I've had access to a DS25 on which I've tested (and debugged) several improvements including:

- Rewrote the TLB handling in the pmap module to fix several MP stability issues and generally improve performance.
- Implemented support for fast-soft-interrupts, which furthers improves performance of the system.
- Some general bug fixing and performance tweaking.
- Fixed entering kernel debugger when MULTIPROCESSOR is enabled.

Out of precaution, I did disable it in the INSTALL kernels, mainly to save a little space.  But the install environment doesn't really benefit from it anyway...

The system has been very stable for me, and is feeling much snappier.  I've done several builds of the system without any problems other than a kernel FP completion bug that I fixed (that bug had been there since Ross Harvey added that code like 20 years ago, but was only recently triggered by an update of awk!).

In the coming weeks, I want to implement support for kernel preemption.  From a "normal use" kernel features perspective, that gets the Alpha port pretty close to back on-par with the x86_86 and aarch64 platforms (kernel modules are currently broken, so I suppose that should go on the list...)

Anyway, I highly recommend folks install the latest -current on their Alpha systems so we can collectively shake out any gremlins that might still be lurking.  Or, you know, to enjoy the other benefits :-). If you want to install pre-built binaries, you might need to wait a few days for official NetBSD daily build to catch up to the latest sources.

Let me know if you run into problems!

-- thorpej