testsuite/gcc.dg/pr84877.c: Add machinery to stabilize stack aligmnent

testsuite/gcc.dg/pr84877.c: Add machinery to stabilize stack aligmnent

This test awkwardly "blinks"; xfails and xpasses apparently
randomly for cris-elf using the "gdb simulator".  On
inspection, I see that the stack address depends on the
number of environment variables, deliberately passed to the
simulator, each adding the size of a pointer.

This test is IMHO important enough not to be just skipped
just because it blinks (fixing the actual problem is a
different task).

I guess a random non-16 stack-alignment could happen for
other targets as well, so let's try and add a generic
machinery to "stabilize" the test as failing, by allocating
a dynamic amount to make sure it's misaligned.  The most
target-dependent item here is an offset between the incoming
stack-pointer value (within main in the added framework) and
outgoing (within "xmain" as called from main when setting up
the p0 parameter).  I know there are other wonderful stack
shapes, but such targets would fall under the "complicated
situations"-label and are no worse off than before.

	* gcc.dg/pr84877.c: Try to make the test result	consistent by
	misaligning the stack.