This fixes linking if built in MSVC mode. Even though the main unwinder interface isn't of much use in an MSVC environment, one might plausibly want to use libunwind for backtraces even there.
Only handled x86, x86_64 and arm64 so far. In particular, on arm32, it's not one function but a number of smaller functions.
Just posting this as a RFC. It simplifies the interface of the assembly somewhat, but might also require changes to the code for the cases where the calling convention differs between C and C++ functions (like on i386 windows).