r600g: Don't build llvm_wrapper.cpp when we aren't using LLVM

r600g: Don't build llvm_wrapper.cpp when we aren't using LLVM

We were using the NEED_RADEON_GALLIUM conditional to decide whether or not
to build llvm_wrapper.cpp, which is required for using the LLVM backend.
llvm_wrapper.cpp needs to be linked against the LLVM IPO libary
and this library is only added to LLVM_LIBS if either opencl or the
r600-llvm-compiler is enabled.

The NEED_RADEON_GALLIUM conditional is set to true when enabling the
radeonsi driver, so if the radeonsi and r600 drivers are enabled without
also enabling opencl or r600-llvm-compiler, llvm_wrapper.cpp will be
built, but the IPO library won't be added to LLVM_LIBS.  This was
causing unresolved symbol errors when buiding with this configuration.

https://bugs.freedesktop.org/show_bug.cgi?id=59831

Tested-by: Alex Deucher <alexander.deucher@amd.com>