Don't crash when compiling for 16-bit platforms.

Reviewed-on: https://github.com/dlang/dmd/pull/10306

gcc/d/ChangeLog:

	* d-target.cc: Include diagnostic.h.
	(Target::_init): Set Tsize_t and Tptrdiff_t as D ushort and short if
	the target pointer size is 2.  Add sorry if the pointer size is not
	either 2, 4, or 8.

From-SVN: r274768