ARM, PR15868 Release-blocker fix. ByVal parameters padding fix.

[Stepan Dyatkovskiy]

Hi all,

In case when stack alignment is 8 and GPRs parameter part size is not 
N*8: we add padding to GPRs part, so part's last byte must be recovered 
at address K*8-1.
We need to do it, since remained (stack) part of parameter starts from 
address K*8, and we need to "attach" "GPRs head" without gaps to it:
Stack:
|---- 8 bytes block ----| |---- 8 bytes block ----| |---- 8 bytes...
[ [padding] [GPRs head] ] [ ------ Tail passed via stack  ------ ...

Note, once we added padding we need to correct *all* Arg offsets that 
are going after padded one.
That's why we need this fix: Arg offsets were never corrected before 
this patch. See new test-case included in patch.

We also don't need to insert padding for byval parameters that are 
stored in GPRs only. We need pad only last byval parameter and only in 
case it outsides GPRs and stack alignment = 8.

Please find the patch in attachment.

This patch reduces stack usage for some cases:
We can reduce ArgRegsSaveArea since inner 4 bytes sized byval params my 
be "packed" with alignment 4.

This patch also fixes PR15868 release-blocking issue.

-Stepan.

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