style: remove trailing whitespace

Result of running 'hg m5style --skip-all --fix-white -a'.

cpu: Timebuf const accessors

Add const accessors for timebuf elements.

Move sched_list.hh and timebuf.hh from src/base to src/cpu.
These files really aren't general enough to belong in src/base.
This patch doesn't reorder include lines, leaving them unsorted
in many cases, but Nate's magic script will fix that up shortly.