[gdbsupport] Fix UB in print-utils.cc:int_string

[gdbsupport] Fix UB in print-utils.cc:int_string

When building gdb with -fsanitize=undefined, I run into:
...
(gdb) PASS: gdb.ada/access_to_packed_array.exp: set logging enabled on
maint print symbols^M
print-utils.cc:281:29:runtime error: negation of -9223372036854775808 cannot \
  be represented in type 'long int'; cast to an unsigned type to negate this \
  value to itself
(gdb) FAIL: gdb.ada/access_to_packed_array.exp: maint print symbols
...

By running in a debug session, we find that this happens during printing of:
...
   typedef system.storage_elements.storage_offset: \
     range -9223372036854775808 .. 9223372036854775807;
...
Possibly, an ada test-case could be created that exercises this in isolation.

The problem is here in int_string, where we negate a val with type LONGEST:
...
         return decimal2str ("-", -val, width);
...

Fix this by, as recommend, using "-(ULONGEST)val" instead.

Tested on x86_64-linux.