fix the type of wchar_t on arm; support wchar_t varying with arch

really wchar_t should never vary, but the ARM EABI defines it as an
unsigned 32-bit int instead of a signed one, and gcc follows this
nonsense. thus, to give a conformant environment, we have to follow
(otherwise L""[0] and L'\0' would be 0U rather than 0, but the
application would be unaware due to a mismatched definition for
WCHAR_MIN and WCHAR_MAX, and Bad Things could happen with respect to
signed/unsigned comparisons, promotions, etc.).

fortunately no rules are imposed by the C standard on the relationship
between wchar_t and wint_t, and WEOF has type wint_t, so we can still
make wint_t always-signed and use -1 for WEOF.

arch/x86_64/bits/wchar.h

@@ -0,0 +1,4 @@
+#ifndef WCHAR_MIN
+#define WCHAR_MIN (-1-0x7fffffff)
+#define WCHAR_MAX (0x7fffffff)
+#endif