Make tst-strtod-underflow type-generic

Make tst-strtod-underflow type-generic

The test tst-strtod-underflow covers various edge cases close to the
underflow threshold for strtod (especially cases where underflow on
architectures with after-rounding tininess detection depends on the
rounding mode).  Make it use the type-generic machinery, with
corresponding test inputs for each supported floating-point format, so
that other functions in the strtod family are tested for underflow
edge cases as well.

Tested for x86_64.