…G_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

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@357711 91177308-0d34-0410-b5e6-96231b3b80d8