optimisation from MacOS X.3: the comm pages

Hello everybody

I upgraded COMPAT_DARWIN to run MacOS X.3 binaries. Apple introduced a
performance hack that broke backward compatibility, but we could
probably setup somethign similar while retaining it.

Two pages (known as the comm pages) are mapped by the kernel at the end
of each process memory space. On PowerPC they can be accessed through an
absolute branch, so they don't need relocations. Theses pages contains
several basic routines optimized for the current CPU: bzero, memcpy, and
so on.

The point is that the process does not has to test the CPU type, it just
uses kernel provided bzero, which is optimized for the hardware.

Other trick: the pages start with values that are really readen often:
ncpu, system time, and so on. That enable user program to perform a
gettimeofday without the overhead of a system call. There are a few
sysctl we call really often, so there is probably an easy win to do
there. 

But if we want to keep backward compatibility, we need a mechanism to
test for this page presence from libc (probably a sysctl). MacOS X.3
binaries just fail if the page is not there.

The implementation in COMPAT_DARWIN can be found in
src/sys/compat/darwin/darwin_commpage.c

Anyone interested by implementing that?
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