[Bug target/15106] Vector varargs failure for AltiVec on ppc32 linux

[Hartmut Penner]

> No, not a full fix.  I keep uncovering more bugs.  :-(  For instance,
> -maltivec -mabi=no-altivec can't work on ppc32 linux since the stack
> is only aligned to 8 bytes, yet altivec loads and stores require 16
> byte alignment.

Same problem on ppc64 (failure with testcase of bug 15106, pass_01,
pass_11).
This combination does not make sense, AltiVec ABI is precondition for
usefull
AltiVec instruction usage. We could issue an error for that case.
      Hartmut