Fix larger than WCHAR_MAX differences in wcs{,n}cmp (#795)

The C standard states:

> Unless explicitly stated otherwise, the functions described in this
> subclause order two wide characters the same way as two integers of
> the underlying integer type designated by wchar_t.
>
> [...]
>
> The wcscmp function returns an integer greater than, equal to, or
> less than zero, accordingly as the wide string pointed to by s1 is
> greater than, equal to, or less than the wide string pointed to by
> s2.
>
> [...]
>
> The wcsncmp function returns an integer greater than, equal to, or
> less than zero, accordingly as the possibly null-terminated array
> pointed to by s1 is greater than, equal to, or less than the
> possibly null-terminated array pointed to by s2.
- C Standard, 7.31.4.4. Wide string comparison functions

Cosmopolitan fails to obey this in cases where the difference between
two wide characters is larger than WCHAR_MAX.

This means that, for example, the following program:

 #include <stdio.h>
 #include <wchar.h>
 #include <limits.h>

int main()
{
    wchar_t str1[] = { WCHAR_MIN, L'\0' };
    wchar_t str2[] = { WCHAR_MAX, L'\0' };
    printf("%d\n", wcscmp(str1, str2));
    printf("%d\n", wcsncmp(str1, str2, 2));
}

will print `1` twice, instead of the negative numbers mandated by the
standard (as WCHAR_MIN is less than WCHAR_MAX)

This patch fixes this, along with the associated Github issue,
https://github.com/jart/cosmopolitan/issues/783

libc/str/wcscmp.c

@@ -30,5 +30,5 @@ int wcscmp(const wchar_t *a, const wchar_t *b) {
   size_t i = 0;
   if (a == b) return 0;
   while (a[i] == b[i] && b[i]) ++i;
-  return (unsigned)a[i] - (unsigned)b[i];
+  return (a[i] > b[i]) - (a[i] < b[i]);
 }

libc/str/wcsncmp.c

@@ -30,5 +30,5 @@ int wcsncmp(const wchar_t *a, const wchar_t *b, size_t n) {
   size_t i = 0;
   if (!n-- || a == b) return 0;
   while (i < n && a[i] == b[i] && b[i]) ++i;
-  return (unsigned)a[i] - (unsigned)b[i];
+  return (a[i] > b[i]) - (a[i] < b[i]);
 }

test/libc/str/strcmp_test.c

@@ -460,11 +460,11 @@ TEST(wcscmp, testTwosComplementBane) {
   EXPECT_EQ(wcscmp(memcpy(B1, "\x00\x00\x00\x80", 4),
                    memcpy(B2, "\x00\x00\x00\x80", 4)),
             0);
-  EXPECT_EQ(-1, wcscmp(memcpy(B1, "\xff\xff\xff\x7f", 4),
+  EXPECT_LT(0, wcscmp(memcpy(B1, "\xff\xff\xff\x7f", 4),
-                       memcpy(B2, "\x00\x00\x00\x80", 4)));
+                      memcpy(B2, "\x00\x00\x00\x80", 4)));
-  EXPECT_EQ(wcscmp(memcpy(B1, "\x00\x00\x00\x80", 4),
+  EXPECT_LT(wcscmp(memcpy(B1, "\x00\x00\x00\x80", 4),
                    memcpy(B2, "\xff\xff\xff\x7f", 4)),
-            1);
+            0);
   free(B2);
   free(B1);
 }
@@ -475,12 +475,12 @@ TEST(wcsncmp, testTwosComplementBane) {
   EXPECT_EQ(wcsncmp(memcpy(B1, "\x00\x00\x00\x80", 4),
                     memcpy(B2, "\x00\x00\x00\x80", 4), 1),
             0);
-  EXPECT_EQ(wcsncmp(memcpy(B1, "\xff\xff\xff\x7f", 4),
+  EXPECT_GT(wcsncmp(memcpy(B1, "\xff\xff\xff\x7f", 4),
                     memcpy(B2, "\x00\x00\x00\x80", 4), 1),
-            -1);
+            0);
-  EXPECT_EQ(wcsncmp(memcpy(B1, "\x00\x00\x00\x80", 4),
+  EXPECT_LT(wcsncmp(memcpy(B1, "\x00\x00\x00\x80", 4),
                     memcpy(B2, "\xff\xff\xff\x7f", 4), 1),
-            1);
+            0);
   free(B2);
   free(B1);
 }