Runtime: Fix bugs in Linux target.

- next occurence absolute time computation failing on 32-bit because of casting
- PLC thread was started before period was set, potentially passing uninitialized timespec to clock_nanosleep()

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot xmlns:xsd="http://www.w3.org/2001/XMLSchema" URI_location="PYRO://127.0.0.1:61284">
  <TargetType/>
  <Libraries Enable_SVGHMI_Library="true"/>
</BeremizRoot>