META_MODE: Fix clang-tblgen flip-flop building after r335707.

The build of this would have MK_LLVM_TARGET_ALL=yes for bootstrap-tools
but MK_LLVM_TARGET_ALL=no for cross-tools and thus would rebuild between
the two.

MFC after: 2 weeks
X-MFC-with: r335707
Sponsored by: Dell EMC