CMake uses the `system_name` parameter by default to determine the .obj file extension.
On the Nuttx platform, our `system_name` is `bare matel`,
while CMake only uses `.o` on Linux/Unix systems otherwise, it uses `.obj`.
Therefore, we will change it to `.o` by default.
Signed-off-by: xuxin19 <xuxin19@xiaomi.com>
