meson: Try to link all-targets module if Gallium OpenCL is enabled
If we don't do this, and we are statically linking LLVM (-Dshared-llvm=disabled) while using a version of LLVM compiled for a general-purpose distribution, then the link fails with undefined references to the functions called by LLVMInitializeAllTargets().
Using a version of LLVM that was built specifically for Mesa will sometimes mask this: if the only backends built into LLVM are backends that we specifically link anyway, then the link will succeed.
According to commit 80817b6e "meson: Adjust Clover's required LLVM modules", all-targets is not available when using CMake to locate LLVM, so make it optional rather than mandatory.
This partially reverts commit 80817b6e.