sampler: Move trapframe decoding out of log
sampler: Move trapframe decoding out of log

Now the log just takes a pmuevent, which means we can construct the
pmuevent using means other than a trapframe.

bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6