Don't use an unqualified 'abs' function call with a builtin type.

This is dangerous for numerous reasons. The primary risk here is with
floating point or double types where if the wrong header files are
included in a strange order this can implicitly convert to integers and
then call the C abs function on the integers. There is a secondary risk
that even impacts integers where if the namespace the code is written in
ever defines an abs overload for types within that namespace the global
abs will be hidden. The correct form is to call std::abs or write 'using
std::abs' for builtin types (and only the latter is correct in any
generic context).

I've also added the requisite header to be a bit more explicit here.
[ADT] Add an (ADL-friendly) abs free function for APFloat that returns
by value having cleared the sign bit.