rt-tests: disable for uclibc mips

rt-tests is not supported by a uclibc toolchain that does not implement
_pid in struct sigevent. Currently this is all MIPS architectures
in uclibc.

Fixes:
  http://autobuild.buildroot.net/results/074/074265602bec4aba6c82d1aee389045e8ad33d4b



Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>