When cross compiling llvm and clang there is a native step where we build
llvm-config llvm-tblgen and clang-tblgen for the host compiler.
When someone in doing this in a stage process they may already have these tools and wish to opt to use the existing ones instead.
This adds support for 3 new optional cmake flags to disable building of the native steps with the expectation of having them on the PATH already.
Stage -1 - install gcc or clang for host
Stage 0 - trust untrust
Stage 1 - build host clang and llvm and cross compile a crt, compiler-rt, libunwind, libcxx etc for a different target than the host using compiler we just built
Stage 2 - cross compile llvm and clang using the new target support added in stage 1 and using the native tools built in stage 1.
These defines come in handy in stage 2, removing the need to build these tools twice.