Change ptid_t::tid to ULONGEST

Change ptid_t::tid to ULONGEST

The ptid_t 'tid' member is normally used as an address in gdb -- both
bsd-uthread and ravenscar-thread use it this way.  However, because
the type is 'long', this can cause problems with sign extension.

This patch changes the type to ULONGEST to ensure that sign extension
does not occur.