Fix float conversion for PPC.

Fix float conversion for PPC.

Origin: backport, https://github.com/abseil/abseil-cpp/commit/c36d825d9a5443f81d2656685ae021d6326da90c

In PPC `long double` is a double-double representation which behaves weirdly
wrt numeric_limits. Don't take `long double` into account when we are not
handling `long double` natively anyway.

Fix the convert test to always run the conversion even if we are not going to
compare against libc's printf result. This allows exercising the code itself to
make sure we don't trigger assertions or UB found by sanitizers.

The author works at Google. Upstream applied this patch as Piper revision
355857729 and exported it to GitHub; the Applied-Upstream URL above points to
the exported commit.

Gbp-Pq: Name ppc-float-conversion.diff