[libc++abi] Default to DLL annotations on Windows

`__declspec(dllexport)` and `__declspec(dllimport)` should only be used
when building libc++abi as a DLL, but that's the more common use case,
so default to adding the annotations and add an option to opt out.

Similar to r282449, which made the corresponding change for libc++.

Differential Revision: https://reviews.llvm.org/D24945

llvm-svn: 282470