cmake: Auto-detect libgcc/rt on Linux
cmake: Auto-detect libgcc/rt on Linux

hioa-cs/IncludeOS,alfred-bratterud/IncludeOS,AndreasAakesson/IncludeOS,AnnikaH/IncludeOS,alfred-bratterud/IncludeOS,AndreasAakesson/IncludeOS,AndreasAakesson/IncludeOS,mnordsletten/IncludeOS,alfred-bratterud/IncludeOS,AndreasAakesson/IncludeOS,mnordsletten/IncludeOS,mnordsletten/IncludeOS,hioa-cs/IncludeOS,AnnikaH/IncludeOS,hioa-cs/IncludeOS,AnnikaH/IncludeOS,mnordsletten/IncludeOS,alfred-bratterud/IncludeOS,AnnikaH/IncludeOS,mnordsletten/IncludeOS,mnordsletten/IncludeOS,AndreasAakesson/IncludeOS,hioa-cs/IncludeOS,hioa-cs/IncludeOS,AndreasAakesson/IncludeOS,alfred-bratterud/IncludeOS,AnnikaH/IncludeOS