target-ppc: fix sNaN propagation

target-ppc: fix sNaN propagation

The current FPU code returns 0.0 if one of the operand is a
signaling NaN and the VXSNAN exception is disabled.

fload_invalid_op_excp() doesn't return a qNaN in case of a VXSNAN
exception as the operand should be propagated instead of a new
qNaN to be generated. Fix that by calling fload_invalid_op_excp()
only for the exception generation (if enabled), and use the softfloat
code to correctly compute the result.

Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>