Debugger: Don't hard code /usr/bin/gdbserver

Just requiring it to be in $PATH is more flexible and helps e.g.
in cases where /usr is physically read-only on the device and
no or an unsuitable gdbserver is present.

Task-number: QTCREATORBUG-8705
Change-Id: I07bb30dbcc55ecc4c89d4dfda8a873197da43604
Reviewed-by: hjk <hjk121@nokiamail.com>