nettle: fix ARM NEON breakage

Nettle's ARM NEON optimizations use some assembly constructs that aren't
friendly with binutils < 2.21, so just disable it for external
toolchains or internal with an unsupported version.
http://autobuild.buildroot.net/results/3c6f91379f90d438b4da3dd330cf89e1dd746aaf/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>