much closer to passing the gcc struct layout tests.

It might be possible to refactor this a bit, but I'm not sure there's 
actually enough common code for that to be useful.

To get the calling convention completely correct, a bit of 
platform-specific code is necessary even for x86-Linux. On x86-Linux, the
alignment of function parameters is extremely strange; as far as I can tell,
it's always 4 except for SSE vectors or structs containing SSE vectors.  I'm 
continuing to investigate this.

llvm-svn: 51839