[SystemZ] Enable long displacement constraints for inline ASM operands

This enables use of the 'S' constraint for inline ASM operands on
SystemZ, which allows for a memory reference with a signed 20-bit
immediate displacement. This patch includes corresponding documentation
and test case updates.

I've changed the 'T' constraint to match the new behavior for 'S', as
'T' also uses a long displacement (though index constraints are still
not implemented). I also changed 'm' to match the behavior for 'S' as
this will allow for a wider range of displacements for 'm', though
correct me if that's not the right decision.

Author: colpell
Differential Revision: http://reviews.llvm.org/D21097

llvm-svn: 272266