Allow llvm-ar in BundleStatic.cmake

[alexreinking]

When building Halide with Halide_BUNDLE_STATIC enabled and with a Clang toolchain, the detected llvm-ar for CMAKE_AR does not print GNU (unsurprisingly), so CMake configuration hits a fatal error:

bundle_static_libs not implemented for the present toolchain

Fortunately, llvm-ar supports the same MRI scripts GNU ar does, so it can follow that code path, too.