testsuite: Remove fragile stack pointer checks

testsuite/libffi.call/closure_stdcall.c and
testsuite/libffi.call/closure_thiscall.c include inline assembly to save
the stack pointer before and after the call, and compare the values.
However, compilers can and do leave the stack in different states for
these two pieces of inline assembly, such as by saving a temporary value
on the stack across the call; observed with gcc -Os, and verified as
spurious through careful inspection of disassembly.