[SystemZ] Handle index-only addresses in (dis)assembler

Most addresses in SystemZ instructions take two registers,
an index register and a base register.  However, either of
those can be omitted.  If there is just a single register,
this usually is taken as the base register - however, there
are certain rare cases where you specifically want to use
an index register but no base register.  This is currently
not handled consistently by the assembler / disassembler.

Fix this by
 - always emitting a dummy 0 as base register for index-
   only addresses
 - correctly handle dummy 0 as indicating no base register
   when parsing an address

This is compatible with current GNU binutils behavior.