relibc: integrate upstream wchar.h include ordering + stdbool.h POSIX fix

Two upstream improvements integrated:

1. wchar.h cbindgen.toml — adopt upstream include ordering:
   - Remove redundant wchar_t redefinition (now provided by bits/wchar-t.h)
   - Define wint_t BEFORE #include <stddef.h> (conflict with GCC __need_wint_t)
   - Drop sys_includes, use no_includes=true (all includes in after_includes)
   - Cleaner circular-dependency breaking (wchar.h → stdio.h → inttypes.h → wchar.h)

2. stdbool.h — fix to POSIX standard:
   - Change 'typedef _Bool bool' to '#define bool _Bool' (C mode)
   - Remove 'typedef bool _Bool' (C++ mode, not in POSIX)
   - Only emit bool/true/false defines in C++ when __cplusplus < 201103L

include/stdbool.h

@@ -2,12 +2,12 @@
 #define _STDBOOL_H
 
 #ifndef __cplusplus
-typedef _Bool bool;
+#define bool _Bool
 #define true 1
 #define false 0
 #else /* __cplusplus */
-typedef bool _Bool;
 #if __cplusplus < 201103L
+#define bool bool
 #define false false
 #define true true
 #endif /*__cplusplus < 201103L*/