Implement code to keep the stack pointer aligned to an 8 byte boundary.

Implement code to keep the stack pointer aligned to an 8 byte boundary.
This improves the performance of the power benchmark by a few percent.
This will be neccesary for SSE code, which requires 16 byte alignment of
the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5320 91177308-0d34-0410-b5e6-96231b3b80d8