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;

test/libc/stdio/snprintf_test.c

@@ -71,3 +71,33 @@ TEST(snprintf, testInf) {
   for (i = 4; i < 10; ++i)
     ASSERT_EQ(buf[i], '\0');
 }
+
+TEST(snprintf, testUppercaseCConversionSpecifier) {
+  char buf[10] = {};
+  int i = snprintf(buf, sizeof(buf), "%C", L'a');
+
+  ASSERT_EQ(i, 1);
+  ASSERT_STREQ(buf, "a");
+
+  i = snprintf(buf, sizeof(buf), "%C", L'☺');
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "☺");
+}
+
+// Make sure we don't va_arg the wrong argument size on wide character
+// conversion specifiers
+TEST(snprintf,
+     testWideCConversionSpecifierWithLotsOfArgumentsBeforeAndOneAfter) {
+  char buf[20] = {};
+  int i = snprintf(buf, sizeof(buf), "%d%d%d%d%d%d%d%d%lc%d", 0, 0, 0, 0, 0, 0,
+                   0, 0, L'x', 1);
+
+  ASSERT_EQ(i, 10);
+  ASSERT_STREQ(buf, "00000000x1");
+
+  memset(buf, 0, sizeof(buf));
+  i = snprintf(buf, sizeof(buf), "%d%d%d%d%d%d%d%d%C%d", 0, 0, 0, 0, 0, 0, 0, 0,
+               L'x', 1);
+  ASSERT_EQ(i, 10);
+  ASSERT_STREQ(buf, "00000000x1");
+}