build: use -mbig-obj for mingw-w64 Debug builds

Windows cross builds using `-O0` currently fail to compile, as some
objects have too many sections. As a convenience, add `-mbig-obj` to
our compile flags when using the `Debug` build type, so that if someone
tries to build this way, it will work.

This would also be needed if we switched the depends flags to -O0.

`-mbig-obj`

> On PE/COFF target this option forces the use of big object
> file format, which allows more than 32768 sections.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

Author: fanquake

CMakeLists.txt

@@ -298,6 +298,15 @@ if(WIN32)
     try_append_linker_flag("-Wl,--major-subsystem-version,6" TARGET core_interface)
     try_append_linker_flag("-Wl,--minor-subsystem-version,2" TARGET core_interface)
   endif()
+
+  # Workaround producing large object files, which cannot be handled by the assembler.
+  # More likely to happen with no, or lower levels of optimisation.
+  # See discussion in https://github.com/bitcoin/bitcoin/issues/28109.
+  if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+    try_append_cxx_flags("/bigobj" TARGET core_interface_debug SKIP_LINK)
+  else()
+    try_append_cxx_flags("-Wa,-mbig-obj" TARGET core_interface_debug SKIP_LINK)
+  endif()
 endif()
 
 # Use 64-bit off_t on 32-bit Linux.