[ARC] Enable indexed loads for elf targers.

[ARC] Enable indexed loads for elf targers.

Enable indexed loads only for elf target, as the linux ones need more testing.

gcc/
2017-02-28  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.opt (mindexed-loads): Use initial value
	TARGET_INDEXED_LOADS_DEFAULT.
	(mauto-modify-reg): Use initial value
	TARGET_AUTO_MODIFY_REG_DEFAULT.
	* config/arc/elf.h (TARGET_INDEXED_LOADS_DEFAULT): Define.
	(TARGET_AUTO_MODIFY_REG_DEFAULT): Likewise.
	* config/arc/linux.h (TARGET_INDEXED_LOADS_DEFAULT): Define.
	(TARGET_AUTO_MODIFY_REG_DEFAULT): Likewise.

From-SVN: r250274