compiler: always use fat function pointers with context

This reduces complexity in the compiler without affecting binary sizes
too much.

Cortex-M0:   no changes
Linux x64:   no changes
WebAssembly: some testcases (calls, coroutines, map) are slightly bigger

src/runtime/panic.go

@@ -2,7 +2,7 @@ package runtime
 
 // trap is a compiler hint that this function cannot be executed. It is
 // translated into either a trap instruction or a call to abort().
-//go:linkname trap llvm.trap
+//go:export llvm.trap
 func trap()
 
 // Builtin function panic(msg), used as a compiler intrinsic.