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.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