[Clang] Let the linker choose shared or static libunwind unless specified

We shouldn't assume that libunwind.so is available. Rather can defer
the decision to the linker which defaults to libunwind.so, but if .so
isn't available, it'd pick libunwind.a. Users can use -static-libgcc
and -shared-libgcc to override this behavior and explicitly choose
the version they want.

Differential Revision: https://reviews.llvm.org/D127528