Make __strtod_internal tests 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 the tests of the internal __strtod_internal interface to cover
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.  As an internal interface, there are no
aliases for different types with the same ABI (however,
__strtold_internal is defined even if long double has the same ABI as
double), so macros used by the type-generic testing code are redefined
as needed to avoid expecting such aliases to be present.

Tested for x86_64.