PR78468 - add alloca alignment test

PR78468 - add alloca alignment test

Add an alignment test to check that aligned alloca's really do get
correctly aligned.  Some targets may not ensure SP is always a multiple
of STACK_BOUNDARY (particularly with outgoing arguments), which means
aligned alloca does not get correctly aligned.  This can be fixed either
by aligning the outgoing arguments or setting STACK_BOUNDARY correctly.

    testsuite/
        PR middle-end/78468
	* gcc.dg/pr78468.c: Add alignment test.

From-SVN: r251811