Heads up: UVM changes

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

I changed UVM to allow for concurrent page faults on shared objects. 
Previously this was single threaded due to locking, which caused a lot of
contention over busy objects like libc.so or PostgreSQL's shared buffer
for example.

This works well for me on amd64, but I haven't tried it on other machines. 
From code inspection I can see that some architectures need more work so
this is disabled on them until I can make the needed changes: hppa, mips,
powerpc and riscv.  It's enabled everywhere else.

I will keep an eye out for any bug reports.

Thank you,
Andrew