Stack unwinding for 32 bit Windows builds

This resolves issue #908.  

Notable changes:
-  On Windows, LLVM integrated assembler emits bad stack unwind tables when segmented stacks are enabled.  However, unwind info directives in the assembly output are correct, so we generate assembly first and then run it through an external assembler, just like it is already done for Android builds.

- Linker is invoked  via "g++" command instead of "gcc": g++ passes the appropriate magic parameters to the linker, which ensure correct registration of stack unwind tables in dynamic libraries.