change wint_t to unsigned

aside from the obvious C++ ABI purpose for this change, it also brings
musl into alignment with the compiler's idea of the definition of
wint_t (use in -Wformat), and makes the situation less awkward on ARM,
where wchar_t is unsigned.

internal code using wint_t and WEOF was checked against this change,
and while a few cases of storing WEOF into wchar_t were found, they
all seem to operate properly with the natural conversion from unsigned
to signed.

arch/arm/bits/alltypes.h.in

@@ -7,7 +7,7 @@ TYPEDEF __builtin_va_list __isoc_va_list;
 #ifndef __cplusplus
 TYPEDEF unsigned wchar_t;
 #endif
-TYPEDEF int wint_t;
+TYPEDEF unsigned wint_t;
 
 TYPEDEF float float_t;
 TYPEDEF double double_t;