Move the alias definition of unw_getcontext to within !defined(__USING_SJLJ_EXCEPTIONS__)

For builds with SJLJ, there is no __unw_getcontext symbol. On Windows,
the weak alias macro also expands to a dllexport directive, which fails
if the symbol doesn't exist.

Differential Revision: https://reviews.llvm.org/D60251

llvm-svn: 357711