set sret attribute as needed on call instructions

Since function pointers do not carry along the function attributes with
them in the type, this needs to be set on the call instruction itself.

Closes #9152
auto merge of #9158 : thestinger/rust/extern, r=alexcrichton

Since function pointers do not carry along the function attributes with
them in the type, this needs to be set on the call instruction itself.

Closes #9152