compiler: fix deferred calls to exported functions

Previously using defer with an exported function generated an invalid function call due to differences between TinyGo's calling convention and the C calling convention.

testdata/calls.go

@@ -72,6 +72,8 @@ func hello(n int) {
 }
 
 func testDefer() {
+	defer exportedDefer()
+
 	i := 1
 	defer deferred("...run as defer", i)
 	i++
@@ -98,6 +100,11 @@ func deferred(msg string, i int) {
 	println(msg, i)
 }
 
+//go:export __exportedDefer
+func exportedDefer() {
+	println("...exported defer")
+}
+
 func testBound(f func() string) {
 	println("bound method:", f())
 }

testdata/calls.txt

@@ -4,6 +4,7 @@ Thing.Print: foo arg: bar
 ...run as defer 3
 ...run closure deferred: 4
 ...run as defer 1
+...exported defer
 loop 3
 loop 2
 loop 1