First step towards varargs support in Mips:

First step towards varargs support in Mips:
- o32 cc must pass all arguments in A0...A3 and stack regardless
if its type (but respect the alignment).
- Store all variable arguments back to the caller stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95500 91177308-0d34-0410-b5e6-96231b3b80d8