[libc++] Use more appropriate conditional for ABI macro definition

The inline function definition ABI macro is gated on COFF dllexport
semantics, so it's more appropriate to mark it with the object file
format macro rather than the generic _WIN32 macro. We now have no uses
of _WIN32 apart from those used to define the other Windows macros :)

Clarify the ABI macro comment and make the object file format check
exhaustive while I'm here.

llvm-svn: 300097