Two changes:

 1) Outline cases for GC pointers from "genPutStructArgStk" into
    their own functions. Helps with readability, will be used in
    the substative portion of the change.
 2) Do not use "Kind::Push" for a case of a struct less than 16
    bytes in size, that does not have GC pointers, on x86. With
    this, we will now always call "genStructPutArgUnroll" for
    the "Unroll" kind. This means "genAdjustStackForPutArgStk"
    has to special-case that. This will go away in the final
    version of the change.

No diffs on x86 or Unix-x64 (the only two affected platforms).

We're not using XMMs on the "push" path.

It is smaller and a little faster than using an XMM
register to first load and then store the struct.