Fix the failing store merging test on Arm.  Aligning variables fixes a
few cases, otherwise disable the test on Arm.  All store merging tests
now pass.  Committed as obvious.

    testsuite/
	* gcc.dg/store_merging_27.c: Fix test for Arm.
	* gcc.dg/store_merging_28.c: Likewise.
	* gcc.dg/store_merging_29.c: Likewise.
	* gcc.dg/tree-ssa/dump-6.c: Likewise.

From-SVN: r273011