Add POSIX's C conversion specifier to printf funcs (#1276)

POSIX specifies the C conversion specifier as being "equivalent to %lc",
i.e. printf("%C", arg) is equivalent in behaviour to printf("%lc", arg).

This patch implements this conversion specifier, and adds a test for it,
alongside another test, which ensures that va_arg uses the correct size,
even though we set signbit to 63 in the code (which one might think will
result in the wrong size of argument being va_arg-ed, but having signbit
set to 63 is in fact what __fmt_stoa expects and is a requirement for it
properly formatting the wchar_t argument - this does not result in wrong
usage of va_arg because the implementation of the c conversion specifier
(which the implementation of the C conversion specifier fallsthrough to)
always calls va_arg with an argument type of int, to avoid the very same
bug occuring with %lc, as the l length modifier also sets signbit to 63)

libc/stdio/fmt.c

@@ -1073,6 +1073,9 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
         }
         break;
       }
+      case 'C':
+        signbit = 63;
+        // fallthrough
       case 'c':
         if ((charbuf[0] = va_arg(va, int))) {
           p = charbuf;