sparc: Do not test preservation of NaN payloads for LEON

The FPU used by LEON does not preserve NaN payload. This change allows
the math/test-*-canonicalize tests to pass on LEON.

Signed-off-by: Daniel Cederman <cederman@gaisler.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>