This patch is to add a new target hook 'to_can_download_tracepoint_loc'
to determine whether it is OK for remote target to receive tracepoint
locations.

Note that, `download_tracepoint_loc' itself can check this in lower
level, but it is not efficient, and it leaves managing location's status
to lower level.

This patch is needed by patch 4/8.

-- 
Yao (齐尧)