Reworking the stack layout that the MicroBlaze backend generates.

Reworking the stack layout that the MicroBlaze backend generates.

The MicroBlaze backend was generating stack layouts that did not
conform correctly to the ABI. This update generates stack layouts
which are closer to what GCC does.

Variable arguments support was added as well but the stack layout
for varargs has not been finalized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97807 91177308-0d34-0410-b5e6-96231b3b80d8