Refactor CMake system to allow cross OS DAC compile

[sdmaclea]

Add CLR_CMAKE_HOST_OS

Add rules to determine which cross OS combinations are valid

Make add_defintions depend on TARGET OS properties

Wherever reasonable make C++ define defintions depend on runtime target
rather than host.

[sdmaclea]

This collided with @am11's #1753. I have fixed the merge conflicts.

[am11]

Currently src/coreclr and src/installer are sharing host definitions via eng/native/configureplatform.cmake. We can make src/libraries/Native/Unix/CMakeLists.txt also depend on it. Then there will be only one file setting CLR_CMAKE_{HOST,TARGET}_PLATFORM_* in the entire repo.

[jkotas]

The build break on musl is likely introduced by these changes. Could you please take a look?

[sdmaclea]

Thanks @jkotas. I found the error. I neglected to set CLR_CMAKE_TARGET_LINUX for alpine and tizen. This disabled /PerfMap support in crossgen and caused the failure.