[libunwind] Provide a way to set '_LIBUNWIND_IS_BAREMETAL' through cmake.

Libunwind uses _LIBUNWIND_IS_BAREMETAL in a lot of places but there is no cmake variable to set it. This patch adds such a variable. It is quite like what LIBCXXABI_BAREMETAL does in libcxxabi.

Reviewed By: compnerd, #libunwind

Differential Revision: https://reviews.llvm.org/D84759