This test covers:
* The callee side, making sure that the structs are correctly loaded into registers when `-Zreg-struct-return` is enabled
* The caller side, making sure that callers do receive returned structs in registers when `-Zreg-struct-return` is enabled
Structs of the size of up to 2 registers (8 bytes) can be returned in registers in x86_32.
Therefore, the tests are done with 3 different struct sizes:
* 2 bytes (register returns should happen)
* 8 bytes (last value where register returns should happen)
* 12 bytes (register returns should not happen even when `-Zreg-struct-return` is enabled)
