Add some very basic support for the Intel32 ABI.

In 32-bit mode, all function arguments are passed on the stack, not in
registers.

This ABI support is not complete or properly tested, but at least it
doesn't try to pass arguments in r8.

cranelift/filetests/isa/intel/abi32.cton

@@ -0,0 +1,20 @@
+; Test the legalization of function signatures.
+test legalizer
+isa intel
+
+; regex: V=v\d+
+
+function %f() {
+    sig0 = (i32) -> i32 native
+    ; check: sig0 = (i32 [0]) -> i32 [%rax] native
+
+    sig1 = (i64) -> b1 native
+    ; check: sig1 = (i32 [0], i32 [4]) -> b1 [%rax] native
+
+    sig2 = (f32, i64) -> f64 native
+    ; check: sig2 = (f32 [0], i32 [4], i32 [8]) -> f64 [%xmm0] native
+
+ebb0:
+    return
+}
+