rt-tests: not available on avr32, too old uClibc

The old uClibc used for the AVR32 architecture does not implement
priority inheritance, therefore rt-tests cannot build.

Fixes:

  http://autobuild.buildroot.org/results/695293d889128c666174d5fe03c5deadbe4d5415/build-end.log



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