graphics/mesa-devel: allow users to disable drivers

• DragonFly cannot use anv without futex (Linux, OpenBSD) or _umtx_op (FreeBSD)
• DragonFly doesn't install libelf from ELF Tool Chain
• powerpc64 cannot use LIBUNWIND as it yet to be ported
• powerpc64 on FreeBSD < 13 cannot use -fuse-ld=lld
• anv, i965, iris only support Intel GPUs on x86
• anv, i965, iris work fine without LLVM support

PR: 246913