Fix bogus store_merging_x.c failures for avr.

Fix bogus store_merging_x.c failures for avr.

Store merging isn't profitable for the avr. Add a new
effective target keyword "store_merge", and turn it on
only if non_strict_align *and* int32plus, as targets with
smallish word sizes are unlikely to profit from this optimization.

gcc/testsuite/ChangeLog

2016-11-03  Senthil Kumar Selvaraj  <senthil_kumar.selvaraj@atmel.com>

	* gcc.dg/store_merging_1.c: Require store_merge.
	* gcc.dg/store_merging_2.c: Likewise.
	* gcc.dg/store_merging_4.c: Likewise.
	* gcc.dg/store_merging_5.c: Likewise.
	* gcc.dg/store_merging_6.c: Likewise.
	* gcc.dg/store_merging_7.c: Likewise.
	* gcc.dg/store_merging_8.c: Likewise.
	* lib/target-supports.exp (check_effective_target_store_merge): New.

From-SVN: r241953