Add DMAP_{MIN,MAX}_PHYSADDR

This allows us to change the size and location if needed later.
These don't build on arm64, skip them for now
Update _setjmp and implement setjmp

We now store & restore the vfp instructions in _setjmp.
Clean up the pmap code a little

Add a new function to check if the entry is dirty. We don't yet track
this correctly so it will return true for all writable pages.

Check if the access flag is set to determine if a page has been accessed.
This will also need to be tracked in software as we are expected to clear
this bit when we create the map and set it in an exception handler when
the access happens.
Force single user mode for now
Copy the amd64 implementation of uiomove_fromphys

This needs pmap changes also copied from amd64.
Update to build msun for arm64

 * The long double type on arm64 is quad-precision
 * Add fenv.c to export the required symbols
 * Fix _fpmath.h to be the correct size and pull out the correct bits
HACK to get llvm building.

These types will be cleaned up when llvm 3.5 is imported
Add the start of the arm64 rtld, the required header
Implement enough to build thread related libraries

The libthread_db part is incorrect as it just calls abort, but it
is enough to build it.
Fix pmap_invalidate_range to always invalidate

Dynamic libraries need this.

While here ass sched_pin/sched_unpin calls around the invalidation
functions. These will be needed when we have SMP to ensure we don't
get moved to a different core during the invalidation process.