MSVC: Restore _DEBUG preprocessor definition in RC debug builds

In commit v3.8.0-rc1~304^2 (MSVC: Do not define _DEBUG explicitly when
using /MDd, 2016-11-15) we removed the `_DEBUG` preprocessor definition
from MSVC C and C++ flags because the `cl` compiler automatically
defines it in Debug builds anyway.  However, the VS generators propagate
C preprocessor definitions to the RC (Windows Resource Compiler) tool.
This means that we used to explicitly define `_DEBUG` for RC debug
builds.  Therefore existing project code may expect the definition to be
there even though the `rc` compiler itself does not implicitly define
`_DEBUG` in debug builds.

Add the `_DEBUG` flag to the default `CMAKE_RC_FLAGS_DEBUG` instead
to restore this definition for RC debug builds.  This also makes it
available consistently in VS, Ninja, and Makefile generators.

Fixes: #16745

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