toolchain/toolchain-external-andes-nds32: disable static build

Buildroot does not support static build with glibc. Since this external
toolchain uses glibc, disable static build with this toolchain.

Fixes:
http://autobuild.buildroot.net/results/3c93cfac81f15f4c18eb7ad578ad86bb7bcf1c12/
http://autobuild.buildroot.net/results/63994f51a2b224b66acfafe5b236249d867a507d/
http://autobuild.buildroot.net/results/96c3be922a96c50fbd8e68059f9ced8a2a75f9ab/



Cc: Nylon Chen <nylon7@andestech.com>
Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>