gcc/target.cmake: fixup for #58408 (fix build with GCC 13)

58408 fixed compilation for GCC >= 13.1. But -dumpversion's output
length depends on compile time configuration. It might only yield the
major version. So use -dumpfullversion instead, which is guaranteed to
always include major, minor and patch version.

Signed-off-by: Jan Henke <Jan.Henke@taujhe.de>