isnan and isinf aren't functions.

isnan was a function in POSIX in 1997, but changed to a macro only in 2004
to align with the C standard. isinf wasn't in POSIX until 2004, where it
appeared as a macro only (but other C libraries already had it as a
function).

Now the C++ standard has added ::isnan and ::isinf functions with different
signatures from the historical ones, so we need to move our historical cruft
out of the way.

We'll keep the implementations for backwards compatibility.

Bug: http://b/34724220
Test: ran tests
Change-Id: Id665f0344af6fe6ed245106e60231f4ef2027f41