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.