Like the original MIPS, HPPA has the MSB of an SNaN set.
However, it has different rules for silencing an SNaN:
(1) msb is cleared and (2) msb-1 must be set if the fraction
is now zero, and (implementation defined) may be set always.
I haven't checked real hardware but chose the set always
alternative because it's easy and within spec.

Signed-off-by: Richard Henderson <rth@twiddle.net>