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

src/test/regress/sql/float8.sql

@@ -40,6 +40,7 @@ SELECT ' INFINITY    x'::float8;
 SELECT 'Infinity'::float8 + 100.0;
 SELECT 'Infinity'::float8 / 'Infinity'::float8;
 SELECT 'nan'::float8 / 'nan'::float8;
+SELECT 'nan'::numeric::float8;
 
 SELECT '' AS five, * FROM FLOAT8_TBL;