Fix 'symbol NT_TIB has not been declared' wcc error

(fix of commit 9483d5b)

Open Watcom (as of v2 2024-11-02 Build) does not define NT_TIB type
in winnt.h file.  The workaround to define own minimal variant of the
structure.

* include/private/pthread_support.h [WOW64_THREAD_CONTEXT_WORKAROUND
&& __WATCOMC__ && GC_WIN32_THREADS] (GC_NT_TIB_s): Define struct; add
comment.
* include/private/pthread_support.h [WOW64_THREAD_CONTEXT_WORKAROUND
&& __WATCOMC__ && GC_WIN32_THREADS] (GC_NT_TIB) Define to
`struct GC_NT_TIB_s` instead of `NT_TIB`.

include/private/pthread_support.h

@@ -45,7 +45,19 @@
 EXTERN_C_BEGIN
 
 #  ifdef WOW64_THREAD_CONTEXT_WORKAROUND
-#    define GC_NT_TIB NT_TIB
+#    if defined(__WATCOMC__) && defined(GC_WIN32_THREADS)
+/* Open Watcom (as of v2 2024-11-02 Build) does not define NT_TIB   */
+/* in winnt.h file.                                                 */
+struct GC_NT_TIB_s {
+  /* struct _EXCEPTION_REGISTRATION_RECORD */ void *ExceptionList; /* unused */
+  PVOID StackBase;
+  PVOID StackLimit;
+  /* The remaining fields are unused. */
+};
+typedef struct GC_NT_TIB_s GC_NT_TIB;
+#    else
+#      define GC_NT_TIB NT_TIB
+#    endif
 #  endif
 
 typedef struct GC_StackContext_Rep {