This fixes a few x86-64 issues that have crept in and broke bootup.

 - Disable -funit-at-a-time. It breaks symbol exports with gcc 3.3.1-hammer.
 - Fix sched_clock to not access HPET.  The new scheduler uses it more
   extensively and it is not mapped early enough.  I opted to just
   disable the HPET access, because even a slightly non monotonous TSC
   should be accurate enough for scheduling purposes. 
 - Fixes for separate objdirs by Arnd Bergmann
 - Fix two warnings that have crept in