[S390x] Fix stack offset size while passing structure as argument.

[saitama951]

when passing the arguments structure by reference we allocate extra un-necessary bytes in the stack (As it should only use 8 bytes for the reference address).

this example depicts when passing a structure of 16 bytes as an argument
before the patch:

18: e3 20 f0 a0 00 24 stg %r2,160(%r15)
1e: e3 30 f0 b0 00 24 stg %r3,176(%r15)

after the patch:

18: e3 20 f0 a0 00 24 stg %r2,160(%r15)
1e: e3 30 f0 a8 00 24 stg %r3,168(%r15)

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[steveisok]

Makes sense to me. @lambdageek, does it look good to you?

[saitama951]

@steveisok can we run the runtime-community CI ?

[lambdageek]

/azp run runtime-community

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[saitama951]

There are no failures reported in s390x CI and other failures are unrelated. is this good to merge?

[lambdageek]

/azp run runtime-community

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[lambdageek]

re-running one more time. there was a hung CI job, and we can't merge on red anymore.