S12Z: GAS: Disallow immediate destination operands

S12Z: GAS: Disallow immediate destination operands

The assembler permitted instructions which attempted to assign to an immediate
operand.  Bizarrely there is a valid machine code for such operations (although
the documentation says it's "inappropriate").  This change causes such attempts
to fail with  an error message.

gas/

	* config/tc-s12z.c (lex_opr): Add a parameter to indicate whether
	immediate mode operands should be permitted.
	* testsuite/s12z/imm-dest.d: New file.
	* testsuite/s12z/imm-dest.l: New file.
	* testsuite/s12z/imm-dest.s: New file.
	* testsuite/s12z/s12z.exp: Add them.