softfloat: fix float{32,64}_maybe_silence_nan() for MIPS

softfloat: fix float{32,64}_maybe_silence_nan() for MIPS

On targets that define sNaN with the sNaN bit as one, simply clearing
this bit may correspond to an infinite value.

Convert it to a default NaN if SNAN_BIT_IS_ONE, as it corresponds to
the MIPS implementation, the only emulated CPU with SNAN_BIT_IS_ONE.
When other CPU of this type are added, this might be updated to include
more cases.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>