rt: Move test functions to rust_test_helpers.cpp
rustc: Use an out pointer to return structs in x86 C ABI. #5347

This Adds a bunch of tests for passing and returning structs
of various sizes to C. It fixes the struct return rules on unix,
and on windows for structs of size > 8 bytes. Struct passing
on unix for structs under a certain size appears to still be broken.
auto merge of #5726 : brson/rust/struct-return, r=brson

r? @nikomatsakis

This doesn't completely fix the x86 ABI for structs, but it does fix some cases. On linux, structs appear to be returned correctly now. On windows, structs are only returned by pointer when they are greater than 8 bytes. That scenario works now.

In the case where the struct is less than 8 bytes our generated code looks peculiar. When returning a pair of u16, C packs both variables into %eax to return them. Our generated code though expects to find one of the pair in %ax and the other in %dx. Similar for u8. I haven't looked into it yet.

There appears to also be struct passing problems on linux, where my `extern-pass-TwoU8s` and `extern-pass-TwoU16s` tests are failing.