s390: larl for Simode on 64-bit

[DJ Delorie]

> So in effect, we have two pointer sizes, 64 being the default, but
> we can also get a 32 bit pointer via the syntax above?  Wow, I'm
> surprised that works.

Yup, been that way for many years.

> And the only time we'd be able to use larl is a dereference of a
> pointer declared with the syntax above.  Right

larl would be used to load the address of an object to *initialize*
such a pointer, but yes.  Regular pointers still use larl but as a
DImode operation.  I.e. larl will always load a 64-bit value into a
register, even if gcc will only use the 32 LSBs.

> OK for the trunk with a simple testcase.  I think you can just scan
> the assembler output for the larl instruction.

Will do, but it's part of a bigger patch.  I just wanted to make sure
there wasn't some side-effect of larl that precluded this use.