softfloat: change default nan definitions to variables

softfloat: change default nan definitions to variables

Most definitions in softfloat.h are really target-independent, but the
file is not because it includes definitions of the default NaN values.
Change those to variables to allow including softfloat.h from files that
are not compiled per-target.  By making them const, the compiler is
allowed to optimize them into softfloat functions that use them.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>