This is simple linear-walk first-fit allocator roughly based on the
allocator in the radeon winsys code.  This allocator has two primary
functional differences:

 1) It cleanly returns 0 on allocation failure

 2) It allocates addresses top-down instead of bottom-up.

The second one is needed for Intel because high addresses (with bit 47
set) need to be canonicalized in order to work properly.  If we allocate
bottom-up, then high addresses will be very rare (if they ever happen).
We'd rather always have high addresses so that the canonicalization code
gets better testing.

v2: - [scott-ph] remove _heap_validate() if NDEBUG is defined (Jordan)

Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>
Tested-by: Scott D Phillips <scott.d.phillips@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>