toolchain/toolchain-external/toolchain-external-andes-nds32: add missing dependencies/select

This external toolchain is pre-built for x86, so it can only work on
x86 and x86-64, and for the latter, the ia32 libraries are necessary.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>