src.opts.mk: switch LLVM_LIBUNWIND to opt-out

PowerPC switched to LLVM_LIBUNWIND along with the switch to Clang/LLVM
in r356111. This leaves only 32-bit Arm and sparc64 not using LLVM's
unwinder, so switch the sense to opt-out.

I elected to list the individual arm MACHINE_ARCHs so future changes
are more clear if LLVM_LIBUNWIND is enabled for one or two but not all
32-bit Arm targets (see PR 233664).