add alignment to enable store merging in strict-alignment targets

add alignment to enable store merging in strict-alignment targets

In g++.dg/opt/store-merging-2.C, the natural alignment of types T and
S is a single byte, so we shouldn't expect store merging on
strict-alignment platforms.  Indeed, without something like the
adjust-alignment pass to bump up the alignment of the automatic
variable, as in GCC 10, the optimization does not occur.

This patch adjusts the test so that the required alignment is
expressly stated, and so we don't rely on its accidentally being there
to get the desired optimization.

for  gcc/testsuite/ChangeLog

	* g++.dg/opt/store-merging-2.C: Add the required alignment.