toolchain: match gcc 'missing-field-initializers' with others

`arcmwdt`` and `clang` both have
`-Wno-missing-field-initializers` in `warning_dw_1` and
`-Wmissing-field-initializers` in `warning_dw_2` while `gcc`
has `-Wmissing-field-initializers` in `warning_dw_1`, so
update it to match.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>