package/mesa3d: disable broken toolchains on nios2

package/mesa3d: disable broken toolchains on nios2

mesa3d configure triggers the _gp bug:
http://autobuild.buildroot.net/results/8be/8be577dae511f82d45ab3b0a9bba370b81ad2ff9//mesa3d-10.6.1/config.log

/home/test/autobuild/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/nios2-linux-gnu/4.8.3/../../../../nios2-linux-gnu/bin/ld: /home/test/autobuild/instance-1/output/host/usr/nios2-buildroot-linux-gnu/sysroot/usr/lib/crt1.o: undefined reference to symbol '_gp'
/home/test/autobuild/instance-1/output/host/usr/nios2-buildroot-linux-gnu/sysroot/usr/lib/libgpg-error.so.0: error adding symbols: DSO missing from command line

Fixes
http://autobuild.buildroot.net/results/8be/8be577dae511f82d45ab3b0a9bba370b81ad2ff9/

[Thomas: propagate the dependency to mesa3d comment about toolchain
dependencies.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>