gdb: rename target_waitstatus_to_string to target_waitstatus::to_string

Make target_waitstatus_to_string a "to_string" method of
target_waitstatus, a bit like we have ptid_t::to_string already.  This
will save a bit of typing.

Change-Id: Id261b7a09fa9fa3c738abac131c191a6f9c13905
8 files changed