unwind: move exported APIs out of header

Ideally, we would do something like inline __declspec(dllexport) to ensure that
the symbol was inlined within libunwind as well as emitted into the final DSO.
This simply moves the definition out of the header to ensure that the *public*
interfaces are defined and exported into the final DSO.

This change also has "gratuitous" code movement so that the EHABI and generic
implementations are co-located making it easier to find them.

The movement from the header has one minor change introduced into the code:
additional tracing to mirror the behaviour of the non-EHABI interfaces.

llvm-svn: 228903