Disable prolog code that aligns the stack when a

Disable prolog code that aligns the stack when a
local object of >16 byte alignment exists.  It does not
work and getting it to work is not trivial, as explained
in the comment.  This fixes all the remaining ppc32
failures in the struct-layout-1 part of the gcc testsuite.

(gcc does not support this either, and the only way to
get such an object is with __attribute__((aligned)) or
generic vectors; it can't be done in a standard-conforming
program, or with Altivec.  So I think disabling it is OK.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48188 91177308-0d34-0410-b5e6-96231b3b80d8