tools/nolibc: fix integer overflow in i{64,}toa_r() and

In twos complement the most negative number can not be negated.

Fixes: b1c21e7d99 ("tools/nolibc/stdlib: add i64toa() and u64toa()")
Fixes: 66c397c4d2 ("tools/nolibc/stdlib: replace the ltoa() function with more efficient ones")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250419-nolibc-ubsan-v2-5-060b8a016917@weissschuh.net

tools/include/nolibc/stdlib.h

@@ -275,7 +275,7 @@ int itoa_r(long in, char *buffer)
 	int len = 0;
 
 	if (in < 0) {
-		in = -in;
+		in = -(unsigned long)in;
 		*(ptr++) = '-';
 		len++;
 	}
@@ -411,7 +411,7 @@ int i64toa_r(int64_t in, char *buffer)
 	int len = 0;
 
 	if (in < 0) {
-		in = -in;
+		in = -(uint64_t)in;
 		*(ptr++) = '-';
 		len++;
 	}