Make tst-strtod2 and tst-strtod5 type-generic

Make tst-strtod2 and tst-strtod5 type-generic

Some of the strtod tests use type-generic machinery in tst-strtod.h to
test the strto* functions for all floating types, while others only
test double even when the tests are in fact meaningful for all
floating types.

Convert tst-strtod2 and tst-strtod5 to use the type-generic machinery
so they test all floating types.  I haven't tried to convert them to
use newer test interfaces in other ways, just made the changes
necessary to use the type-generic machinery.

Tested for x86_64.