poco: fix nios2 fenv build failure

Disable FP environment on nios2.

Fixes:
http://autobuild.buildroot.net/results/3ce/3ce9d6dc5c4e8fceebaa461e5085da7f33fda8a1/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>