package/rt-tests: disable for too old toolchains

package/rt-tests: disable for too old toolchains

rt-tests doesn't build with some "old" toolchains due to missing local
syscall number definition for the new kernel deadline scheduler on some
architectures. Upsteam added __NR_sched_setattr and __NR_sched_setattr
definition only for common architectures (x86, x86_64, arm) and tile if
not already defined [1].

Instead of adding missing syscall number, avoid building rt-tests
with toolchain compiled with too old kernel headers (i.e
!BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_14)

At runtime, a 3.14 kernel must be used otherwise these syscall are not
available.

[1]
https://kernel.googlesource.com/pub/scm/utils/rt-tests/rt-tests.git/+/ef2dee4232b7cae594ef7ecb7230c4bd18cea3f3

Fixes:
http://autobuild.buildroot.net/results/6fa/6facaeb10588bdf3ff029b882a8ae6ffba815cdc

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>