Dynamic pcpu, arm, mips, powerpc, sun, etc. help needed

[Jeff Roberson]

http://people.freebsd.org/~jeff/dpcpu.diff

This patch implements dynamic per-cpu areas such that kernel code can do 
the following in a header:

DPCPU_DECLARE(uint64_t, foo);

and this in source:

DPCPU_DEFINE(uint64_t, foo) = 10;

local = DPCPU_GET(foo);
DPCPU_SET(foo, 11);

The dynamic per-cpu area of non-local cpus is accessable via 
DPCPU_ID_{GET,SET,PTR}.

If you provide an initializer as I used above that will be the default 
value when all cpus come up.  Otherwise it defaults to zero.  This is 
presently slightly more expensive than PCPU but much more flexible. 
Things like id and curthread should stay in PCPU forever.

I had to change the pcpu_init() call on every architecture to pass in 
storage for the dynamic area.  I didn't change the following three calls 
because it wasn't immediately obvious how to allocate the memory:

./powerpc/booke/machdep.c:      pcpu_init(pc, 0, sizeof(struct pcpu));
./mips/mips/machdep.c:  pcpu_init(&__pcpu[0], 0, sizeof(struct pcpu));
./mips/mips/machdep.c:  pcpu_init(pcpup, 0, sizeof(struct pcpu));


I have not tested anything other than amd64.  If you have a !amd64 
architecture, in particular any of the embedded architectures, I would 
really appreciate it.  Some of the arm boards postincrement the end 
address to allocate early memory and some pre-decriment.  Hopefully I got 
it right.

Thanks,
Jeff