libm symbol cleanup.

Some of this code is used in the NDK libandroid_support now, as a static
library, so just being HIDDEN in the ELF sense isn't sufficient.

Rename digittoint to __libm_digittoint so we don't trample anyone's toes.

Also remove imprecise_powl and imprecise_tgammal. It turns out (to my
surprise) that we don't even have ld128 implementations of powl and tgammal,
so even LP64 was just using the "fake_long_double.c" hack in effect. Since
that's the case, let's *actually* do that because then we're not polluting
with the internal names in addition to the aliases.

Bug: N/A
Test: readelf
Change-Id: I273cc8fdc7ce53f9b8dfd4ef7796e358fe901837