[ARM] 3758/1: Preserve signalling NaNs in conversion

Patch from Daniel Jacobowitz

The fcvtds and fcvtsd instructions were generating a qnan bit pattern
for both quiet and signalling NaNs.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>