Revert "isnan and isinf aren't functions."

While this change is correct, GNU libstdc++ 4.9 isn't able to handle a
standard compliant C library. Its <cmath> will `#undef isnan` from
math.h and only adds the function overloads to the std namespace,
making it impossible to use both <cmath> (which gets included by a
lot of other standard headers) and ::isnan.

We're going to have to revert this until we can start turning down
support for gnustl.

This reverts commit e76ee993.

Bug: https://code.google.com/p/android/issues/detail?id=271629
Test: make checkbuild
Change-Id: I394f50271430e78ab801d85c3ee4e87019eda6af