gcc: Use DWARF-4 for Zephyr

Build Zephyr debug info in DWARF-4 format instead of DWARF-5.

The Zephyr test suite uses tools that can't handle DRAWF-5 properly yet (py-elfutils), causing tests to fail if Zephyr has been built with DWARF-5.

Signed-off-by: Bernhard Rosenkränzer bernhard.rosenkraenzer.ext@huawei.com