Improve default cross compiler state

[directhex]

There are several cases where a "basic" build with Mono results in an inscrutable error:

  CMake Error at /usr/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
    Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)

Counter-intuitively, this is typically triggered by missing compiler flags in the BuildMonoAOT build target - caused by poor handling of cases where these variables are not specified on the command line. We also had some incorrect conditionals in variable setting based on or sometimes evaluating the wrong groups.

This PR fixes e.g.:

docker run -e ROOTFS_DIR=/crossrootfs/x64 -it -w $(pwd) --platform linux/amd64 -v $(pwd):$(pwd) mcr.microsoft.com/dotnet-buildtools/prereqs:cbl-mariner-2.0-cross-amd64 ./build.sh -ci -arch x64 --cross -os linux -s mono+libs+host+packs -c Release /p:MonoAOTEnableLLVM=true /p:MonoBundleLLVMOptimizer=true /p:BuildMonoAOTCrossCompiler=true /p:MonoLibClang="/usr/local/lib/libclang.so.16"

But we do not yet have a solution to:

docker run -e ROOTFS_DIR=/crossrootfs/arm64 -it -w $(pwd) --platform linux/amd64 -v $(pwd):$(pwd) mcr.microsoft.com/dotnet-buildtools/prereqs:cbl-mariner-2.0-cross-arm64 ./build.sh -ci -arch arm64 --cross -os linux -s mono+libs+host+packs -c Release /p:MonoAOTEnableLLVM=true /p:MonoBundleLLVMOptimizer=true /p:BuildMonoAOTCrossCompiler=true /p:MonoLibClang="/usr/local/lib/libclang.so.16"

The second case fails because we need a different ROOTFS_DIR value for BuildMonoRuntime and BuildMonoCross targets (we can't build a cross-compiler to cross-compile for arm64 on an x64 host unless we have an x64 crossrootfs for an x64 mono-aot-cross binary)