Don't assume that short is shorter than int.

Don't assume that short is shorter than int.
* gdb.base/exprs.exp ("print unsigned short == (~0)"): Don't
assume that shorts are smaller than ints.  On a 16-bit machine,
this isn't true.
("print unsigned char == (~0)"): Add test that verifies that ~0,
an int, is not equal to ~0 stored in an unsigned char.  This tests
the same thing that the previous test meant to, but works on
16-bit machines, too.
("print unsigned char != (~0)"): Same test, complemented.