Do not store R31 into the caller's link area on PPC.

This violates the ABI (that area is "reserved"), and
while it is safe if all code is generated with current
compilers, there is some very old code around that uses
that slot for something else, and breaks if it is stored
into.  Adjust testcases looking for current behavior.
I've verified that the stack frame size is right in all
testcases, whether it changed or not.  7311323.

llvm-svn: 89811