package/libnss: fix powerpc altivec build failure

package/libnss: fix powerpc altivec build failure

NSS_DISABLE_ALTIVEC variable has been introduced into libnss so let's
use it to prevent Altivec build failure on PowerPc by passing
NSS_DISABLE_ALTIVEC=1 if BR2_POWERPC_CPU_HAS_ALTIVEC is not 'y'.

Fixes:
http://autobuild.buildroot.net/results/957/957cec911bcd68a18418ad02f13e7e3001521c59/
http://autobuild.buildroot.net/results/6a1/6a1578619a477e1605fe152070f004b662f1d839/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>