tools/nolibc: __sysret: support syscalls who return a pointer

No official reference states the errno range, here aligns with musl and
glibc and uses [-MAX_ERRNO, -1] instead of all negative ones.

- musl: src/internal/syscall_ret.c
- glibc: sysdeps/unix/sysv/linux/sysdep.h

The MAX_ERRNO used by musl and glibc is 4095, just like the one nolibc
defined in tools/include/nolibc/errno.h.
Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/ZKKdD%2Fp4UkEavru6@1wt.eu/Suggested-by: David Laight <David.Laight@ACULAB.COM>
Link: https://lore.kernel.org/linux-riscv/94dd5170929f454fbc0a10a2eb3b108d@AcuMS.aculab.com/Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

tools/include/nolibc/sys.h

@@ -28,13 +28,20 @@
 #include "errno.h"
 #include "types.h"
 
-/* Syscall return helper, set errno as -ret when ret < 0 */
+
+/* Syscall return helper for library routines, set errno as -ret when ret is in
+ * range of [-MAX_ERRNO, -1]
+ *
+ * Note, No official reference states the errno range here aligns with musl
+ * (src/internal/syscall_ret.c) and glibc (sysdeps/unix/sysv/linux/sysdep.h)
+ */
+
 static __inline__ __attribute__((unused, always_inline))
-long __sysret(long ret)
+long __sysret(unsigned long ret)
 {
-	if (ret < 0) {
-		SET_ERRNO(-ret);
-		ret = -1;
+	if (ret >= (unsigned long)-MAX_ERRNO) {
+		SET_ERRNO(-(long)ret);
+		return -1;
 	}
 	return ret;
 }