[marshal] Cleanup dead ifdefs (#78435)

1. The `DISABLE_NONBLITTABLE` define was a previous attempt at partitioning the marshaling into a simple version that just supports blittable types and the full legacy marshaller.  It was never used in production.

2. The `ENABLE_ILGEN` was used previously in configurations where runtime codegen was not available to save some space by excluding support for emitting IL at runtime (needed for marshaling, fast managed allocators in sgen, etc).  It also used to emit the ilgen support code into a separate static library that could optionallly be linked into the runtime, and enabled with an appropriate API call (`mono_marshal_ilgen_init`).  This define is now on by default and the API function is a no-op.

3. Mark a collection of `mono_win32_compat_` functions with `MONO_RT_EXTERNAL_ONLY` and move them to `external-only.c` (these were used in mono/mono dllmap support to provide some common kernel32 functions on non-Windows)

4. Remove `MonoMarshalILgenCallbacks` in `marshal-ilgen.c` - just do direct calls to the `_ilgen` helper functions

Fixes #78422

src/mono/CMakeLists.txt

@@ -20,9 +20,6 @@ set(CMAKE_CXX_FLAGS_CHECKED "")
 set(CMAKE_EXE_LINKER_FLAGS_CHECKED "")
 set(CMAKE_SHARED_LINKER_FLAGS_CHECKED "")
 
-# This is always enabled now, and we should go through the code to remove the case here it is not defined
-set(ENABLE_ILGEN 1)
-
 if(NOT MONO_LIB_NAME)
   set(MONO_LIB_NAME "monosgen-2.0")
 endif()

src/mono/cmake/config.h.in

@@ -260,9 +260,6 @@
 /* Disable the interpreter. */
 #cmakedefine DISABLE_INTERPRETER 1
 
-/* Some VES is available at runtime */
-#cmakedefine ENABLE_ILGEN 1
-
 /* Disable non-blittable marshalling */
 #cmakedefine DISABLE_NONBLITTABLE
 

src/mono/mono/component/CMakeLists.txt

@@ -93,8 +93,6 @@ list(APPEND components_for_aot
 set(${MONO_MARSHAL_ILGEN_COMPONENT_NAME}-sources
   ${MONO_COMPONENT_PATH}/marshal-ilgen.c
   ${MONO_COMPONENT_PATH}/marshal-ilgen.h
-  ${MONO_COMPONENT_PATH}/marshal-ilgen-noilgen.c
-  ${MONO_COMPONENT_PATH}/marshal-ilgen-noilgen.h  
 )
 
 # For every component not build into the AOT compiler, build the stub instead

src/mono/mono/component/marshal-ilgen-internals.h

@@ -3,7 +3,4 @@
 
 #include "mono/component/marshal-ilgen.h"
 
-void
-mono_install_marshal_callbacks_ilgen (MonoMarshalILgenCallbacks *cb);
-
-#endif // __MARSHAL_ILGEN_INTERNALS_H__
+#endif // __MARSHAL_ILGEN_INTERNALS_H__