[compiler-rt][CMake] Set --unwindlib=none when using LLVM libunwind

We already link libunwind explicitly so avoid trying to link toolchain's
default libunwind which may be missing. This matches what we already do
for libcxx and libcxxabi.

Differential Revision: https://reviews.llvm.org/D129472

GitOrigin-RevId: 6699f554880087106a02be1675395bf2cc6f46c8
12 files changed