Fix GDB issues with sdk-build MinGW

[stephanosio]

Fix whatever was wrong with GDB and MinGW in 5423f49

[stephanosio]

@nashif Do you happen to remember what was wrong with GDB and sdk-build MinGW at the time of 5423f49? Nevermind, figured.

[stephanosio]

Full log: https://github.com/zephyrproject-rtos/sdk-ng/actions/runs/16422471894/job/46404404386#step:13:395

Error:     /opt/mingw-w64-win32/x86_64-w64-mingw32/include/c++/13.2.0/bits/std_mutex.h:164:5: error: '__gthread_cond_t' does not name a type; did you mean '__gthread_once_t'?
...
Error:     /opt/mingw-w64-win32/x86_64-w64-mingw32/include/c++/13.2.0/condition_variable:83:13: error: '__gthread_cond_t' does not name a type; did you mean '__gthread_once_t'?
...

[stephanosio]

In summary, since GCC 13, libstdc++ supports for std::mutex and std::condition_variable with the win32 thread model when _WIN32_WINNT >= 0x0600 (Vista/Server 2008 or later); but, GDB sets _WIN32_WINNT=0x0501 by default and, in spite of being unsupported, attempts to use these synchronisation primitives -- autoconf does detect this, but it seems like their usage is not properly guarded.

For our purposes, the simplest solution would be to set _WIN32_WINNT to 0x0600 and target Windows Vista/Server 2008 or later since we absolutely have no plans to support Windows XP/Server 2003 or older ...

For more details, see the GCC and GDB bug reports regarding this issue:

• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108225
• https://sourceware.org/bugzilla/show_bug.cgi?id=29966