Insert a hack into get_float8_nan (both core and ecpg copies) to deal with

Insert a hack into get_float8_nan (both core and ecpg copies) to deal with
the fact that NetBSD/mips is currently broken, as per buildfarm member pika.

Also add regression tests to ensure that get_float8_nan and get_float4_nan
are exercised even on platforms where they are not needed by
float8in/float4in.

Zoltán Böszörményi and Tom Lane