place_and_route: Generate textproto timing results

[mczyz-antmicro]

During analysis of outputs of the bazel_rules_hdl/place_and_route step, I noticed room for improvement. Similarly to how the generate_power_results and generate_area_results are used in the global_routing.tcl, the timing report could also be converted to the .textproto format. In particular, outputs of "report_wns", "report_tns" could be saved.

By the way, there is a typo in line, should be "magnitude".