Existing gdb target hook to_download_tracepoint is about downloading
tracepoint on tracepoint level, rather than location level.  However,
locations of one tracepoint may change during inferior run, so we need a
fine-granularity target hook on tracepoint locations, instead of tracepoint.

This patch is to refactor to_download_tracepoint to
to_download_tracepoint_loc.  Beside this refactor, a return value is
added in new hook to reflect whether tracepoint location is
downloaded/installed successfully.  Functionality of gdb is not changed.

-- 
Yao (齐尧)