configure: Disable (clang) initializer-overrides warnings

Disable clang's initializer-overrides warnings, as QEMU makes significant
use of the pattern of initializing an array with a range-based default
entry like
    [0 ... 0x1ff] = { GPIO_NONE, 0 }
followed by specific entries which override that default, and clang
would otherwise warn "initializer overrides prior initialization of
this subobject" when it encountered the specific entry.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
