Don't use -static-libgcc on Windows

It has a tendency to break exceptions, as only TDM-GCC has patches to
make it work to use a .dll while statically linking libgcc.

While in the CLANG64 environment, this is not an issue, the upside of it
is minimal: It only statically links libunwind instead. While libgcc_s
contains some things that aren't always needed, what is provided by
libunwind is always needed.