Implement using "base addresses" in insn operands as default.

Implement using "base addresses" in insn operands as default.

	* config/mmix/mmix.c (mmix_conditional_register_usage): if
	-mabi=gnu, modify fixed_regs to fit the GNU ABI.
	(mmix_extra_constraint): Use 'R' to indicate that GETA should be
	used to read the rtx value.
	(mmix_target_asm_function_epilogue): Fix spacing.
	(mmix_constant_address_p): Handle TARGET_BASE_ADDRESSES.
	(mmix_legitimate_address): Ditto.
	(mmix_encode_section_info): Set SYMBOL_REF_FLAG on rtx:es that
	should be loaded with a GETA insn.  Don't allocate needless extra
	char for nul termination and fix misleading comment.
	(mmix_print_operand_address): Handle constants if
	TARGET_BASE_ADDRESSES.
	(mmix_output_register_setting): Use base addressing if
	TARGET_BASE_ADDRESSES and the number of insns is 3.
	* config/mmix/t-mmix (MULTILIB_EXTRA_OPTS): New.
	* config/mmix/mmix.md ("movdi"): Change the alternative with GETA
	to use R as constraint, add LDA to match s.
	* config/mmix/mmix.h (TARGET_BASE_ADDRESSES): New.
	(TARGET_DEFAULT): Add TARGET_MASK_BASE_ADDRESSES.
	(TARGET_SWITCHES): Add -mbase-addresses, -mno-base-addresses.
	(FIXED_REGISTERS): Make registers $231..$246 fixed by default.
	(MMIX_MMIXWARE_ABI_REG_ALLOC_ORDER): Move $231..$246 last, in
	order with other fixed registers.
	(MMIX_GNU_ABI_REG_ALLOC_ORDER): Put forward $231, in order with
	other parameter/call-clobbered registers.
	* doc/invoke.texi (Option Summary) <MMIX Options>: Add
	-mbase-addresses, -mno-base-addresses.
	(MMIX Options): Ditto.

From-SVN: r49536