This commit improves ABI support in Winch's trampolines mainly by:
* Adding support for the `fastcall` calling convention.
* By storing/restoring callee-saved registers.
One of the explicit goals of this change is to make tests available in the x86_64 target
as a whole and remove the need exclude the windows target.
This commit also introduces a `CallingConvention` enum, to better
reflect the subset of calling conventions that are supported by Winch.