unwind: remove last instance of -Wexpansion-to-defined

This unifies the definition of _LIBUNWIND_BUILD_SJLJ_APIS.  It also further
generalises the definition to allow building these APIs on non-Apple targets
when `-fsjlj-excceptions` is used.  The header is now clean of macros which
expand to defined checks.

llvm-svn: 267509