meson/gallium: Add an option to not use LLVM for gallium draw module

We'd like to use one Mesa build environment which builds our CL compiler stack (which needs Clang/LLVM) and which builds our GL driver. The GL driver doesn't really need LLVM support, and since we're statically linking LLVM, removing it from the driver drastically reduces our DLL size on disk (70%+ size reduction). For our arm64 devices, we currently ship 4 architectures in a single package (x86, x64, arm32, arm64), so this savings adds up quite a bit.

Setting this to false will disable LLVMPipe from being used with swrast, and will fail with SWR. Radeon drivers, Clover, and the microsoft/clc compiler can still use LLVM.