[libc++] Remove Fuchsia-specific knowledge to pick the ABI version

Summary:
The ABI version used by libc++ is a configuration option just like any other
configuration option. It is a knob that can be used by vendors to customize
the libc++ that they ship. As such, we should not be hardcoding vendor-specific
configuration choices in libc++.

When building libc++ for Fuchsia, Fuchsia's build scripts should simply define
the libc++ ABI version to 2 -- this will result in the _LIBCPP_ABI_VERSION
macro being defined in the __config header that is generated when libc++ is
built and installed, which is the correct way to customize libc++'s behavior
for specific vendors.

Reviewers: phosek, EricWF

Subscribers: mgorny, christof, dexonsmith, cfe-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52397

llvm-svn: 343079