x86-64 Windows fastcall ABI support.

This adds support for the "fastcall" ABI, which is the native C/C++ ABI
on Windows platforms on x86-64. It is similar to but not exactly like
System V; primarily, its argument register assignments are different,
and it requires stack shadow space.

Note that this also adjusts the handling of multi-register values in the
shared ABI implementation, and with this change, adjusts handling of
`i128`s on *both* Fastcall/x64 *and* SysV/x64 platforms. This was done
to align with actual behavior by the "rustc ABI" on both platforms, as
mapped out experimentally (Compiler Explorer link in comments). This
behavior is gated under the `enable_llvm_abi_extensions` flag.

Note also that this does *not* add x64 unwind info on Windows. That will
come in a future PR (but is planned!).

cranelift/codegen/src/machinst/valueregs.rs

@@ -175,11 +175,13 @@ impl<R: Clone + Copy + Debug + PartialEq + Eq + InvalidSentinel> ValueRegs<R> {
 }
 
 /// Create a writable ValueRegs.
+#[allow(dead_code)]
 pub(crate) fn writable_value_regs(regs: ValueRegs<Reg>) -> ValueRegs<Writable<Reg>> {
     regs.map(|r| Writable::from_reg(r))
 }
 
 /// Strip a writable ValueRegs down to a readonly ValueRegs.
+#[allow(dead_code)]
 pub(crate) fn non_writable_value_regs(regs: ValueRegs<Writable<Reg>>) -> ValueRegs<Reg> {
     regs.map(|r| r.to_reg())
 }