* It takes 17 fixed arguments for (threaded) PPC to fail. Test
for up to 33 arguments now.
* Also, add some comments to explain why SP shouldn't be set to its
final value eagerly when the stack frame is smaller than the fixed
arguments: accessing slots past SP is a bad idea when interrupts
could hit and overwrite these values. In such cases, leave SP
pointing to the end of the source vector, and only move it back to
the end of the destination vector after the copy loop.