windows fastcall (x64) call convention (#314)

* initial set of work for windows fastcall (x64) call convention

- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
  The shadow space available before the arguments (range 0..32)
  is not used as spill space yet.

* address review feedback

cranelift/filetests/isa/x86/windows_fastcall_x64.cton

@@ -0,0 +1,34 @@
+test compile
+set is_64bit
+set opt_level=best
+set is_pic
+isa x86 haswell
+
+; check if for one arg we use the right register
+function %one_arg(i64) windows_fastcall {
+ebb0(v0: i64):
+    return
+}
+; check: function %one_arg(i64 [%rcx], i64 fp [%rbp]) -> i64 fp [%rbp] windows_fastcall {
+; nextln: ss0 = incoming_arg 16, offset -48
+
+; check if we still use registers for 4 arguments
+function %four_args(i64, i64, i64, i64) windows_fastcall {
+ebb0(v0: i64, v1: i64, v2: i64, v3: i64):
+    return
+}
+; check: function %four_args(i64 [%rcx], i64 [%rdx], i64 [%r8], i64 [%r9], i64 fp [%rbp]) -> i64 fp [%rbp] windows_fastcall {
+
+; check if float arguments are passed through XMM registers
+function %four_float_args(f64, f64, f64, f64) windows_fastcall {
+ebb0(v0: f64, v1: f64, v2: f64, v3: f64):
+    return
+}
+; check: function %four_float_args(f64 [%xmm0], f64 [%xmm1], f64 [%xmm2], f64 [%xmm3], i64 fp [%rbp]) -> i64 fp [%rbp] windows_fastcall {
+
+; check if we use stack space for > 4 arguments
+function %five_args(i64, i64, i64, i64, i64) windows_fastcall {
+ebb0(v0: i64, v1: i64, v2: i64, v3: i64, v4: i64):
+    return
+}
+; check: function %five_args(i64 [%rcx], i64 [%rdx], i64 [%r8], i64 [%r9], i64 [32], i64 fp [%rbp]) -> i64 fp [%rbp] windows_fastcall {