target-alpha: use new float64_unordered_quiet() function

Use float64_unordered_quiet() in helper_cmptun() instead of doing the
the comparison manually.

According to the "Alpha Compiler Writer's Guide", we should use the
_quiet version here, as CMPTUN and CMPTEQ should generate InvalidOp
for SNaNs but not for QNaNs.

Thanks to Peter Maydell <peter.maydell@linaro.org> and Richard
Henderson <rth@twiddle.net> for digging into the manuals.

Acked-by: Richard Henderson  <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
