Support WINDOWS_EXPORT_ALL_SYMBOLS with .def files

The `WINDOWS_EXPORT_ALL_SYMBOLS` target property exports all symbols
found in object files explicitly given to the linker.  However, the
linker may also find additional symbols in dependencies and copy them
into the linked binary (e.g. from `msvcrt.lib`).  Provide a way to
export an explicit list of such symbols by adding a `.def` file as a
source file.

Fixes: #16473

Tests/ModuleDefinition/split_dll.c

@@ -0,0 +1,9 @@
+int split_dll_1(void)
+{
+  return 0;
+}
+
+int split_dll_2(void)
+{
+  return 0;
+}