Fix mono compilation

- Mono defines HOST_WIN32, not CLR_CMAKE_HOST_WIN32
- Follow same pattern from src\native\eventpipe\CMakeLists.txt

Author: GrabYourPitchforks

src/native/external/zlib.cmake

@@ -31,7 +31,7 @@ set(ZLIB_SOURCES_BASE
 
 # enable custom zlib allocator
 add_definitions(-DMY_ZCALLOC)
-if(CLR_CMAKE_HOST_WIN32)
+if(HOST_WIN32 OR CLR_CMAKE_TARGET_WIN32)
     set(ZLIB_SOURCES_BASE ${ZLIB_SOURCES_BASE} ../../libs/System.IO.Compression.Native/zlib_allocator_win.c)
 else()
     set(ZLIB_SOURCES_BASE ${ZLIB_SOURCES_BASE} ../../libs/System.IO.Compression.Native/zlib_allocator_unix.c)