gdb: change store_waitstatus to return a target_waitstatus by value

store_waitstatus is basically a translation function between a status
integer and an equivalent target_waitstatus object.  It would make sense
for it to take the integer as a parameter and return the
target_waitstatus by value.  Do that, and rename to
host_status_to_waitstatus.  Users can then do:

  ws = host_status_to_waitstatus (status)

which does the right thing, given the move constructor of
target_waitstatus.

Change-Id: I7a07d59d3dc19d3ed66929642f82f44f3e85d61b