Since we're generating stubs by hands we don't follow the ABI and don't
create a register spill area.
Don't use this area in compilation callback!

llvm-svn: 77968

Unbreak Win64 CC. Step one: honour register save area, fix some alignment and provide a different set of call-clobberred registers.

llvm-svn: 77962

llvm-svn: 48707

llvm-svn: 48706