toolchain: disable C++ support on AVR32 gcc 4.2 with uClibc

In 25c31600 ("uClibc 0.9.31: remove stray kludges/conditions"), a
conditional that prevents the combination gcc 4.2-avr / C++ / locale
with uClibc 0.9.31 was removed. However, it turns out that the same
problem is affecting uClibc 0.9.33, described at
http://comments.gmane.org/gmane.comp.lib.uclibc.buildroot/24260

.

Therefore, we prevent the combination of gcc 4.2-avr / C++ / locale to
happen.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>