gotypes: fix argument size for signatures without return types (#212)

This fixes a bug in argument size calculation in the case where the function
has no return values. Previously it was padding the argument struct to max
alignment, but this only happens if there are return values following.

Updates #191

tests/fixedbugs/issue191/asm.go

@@ -0,0 +1,15 @@
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	TEXT("Uint16", 0, "func(n uint16)")
+	Doc("Uint16 tests argument size without return types.")
+	RET()
+	Generate()
+}

tests/fixedbugs/issue191/doc.go

@@ -0,0 +1,4 @@
+// Package issue191 tests for correct argument size for a function taking a
+// single uint16 argument. Prior to the fix, this test case triggered an asmdecl
+// error.
+package issue191

tests/fixedbugs/issue191/issue191.s

@@ -0,0 +1,5 @@
+// Code generated by command: go run asm.go -out issue191.s -stubs stub.go. DO NOT EDIT.
+
+// func Uint16(n uint16)
+TEXT ·Uint16(SB), $0-2
+	RET

tests/fixedbugs/issue191/issue191_test.go

@@ -0,0 +1,9 @@
+package issue191
+
+import "testing"
+
+//go:generate go run asm.go -out issue191.s -stubs stub.go
+
+func TestUint16(t *testing.T) {
+	Uint16(42)
+}

tests/fixedbugs/issue191/stub.go

@@ -0,0 +1,6 @@
+// Code generated by command: go run asm.go -out issue191.s -stubs stub.go. DO NOT EDIT.
+
+package issue191
+
+// Uint16 tests argument size without return types.
+func Uint16(n uint16)