Don't enable _LIBUNWIND_BUILD_ZERO_COST_APIS if building the SJLJ APIs

Otherwise, a shared library build with SJLJ APIs enabled would
end up with duplicate symbols.

This didn't occur for the apple && arm case due to specifically
checking for that in the surrounding ifdef.

Differential Revision: https://reviews.llvm.org/D42555

llvm-svn: 323499