[gdb/exp] Fix UB in scalar_binop

When building gdb with -fsanitize=undefined, I run into:
...
$ gdb -q -batch -ex "p -(-0x7fffffffffffffff - 1)"
src/gdb/valarith.c:1385:10: runtime error: signed integer overflow: \
  0 - -9223372036854775808 cannot be represented in type 'long int'
$1 = -9223372036854775808
...

Fix this by performing the substraction in scalar_binop using unsigned types.

Tested on x86_64-linux.