wasm: add //go:wasmexport support (#4451)

This adds support for the `//go:wasmexport` pragma as proposed here:
https://github.com/golang/go/issues/65199

It is currently implemented only for wasip1 and wasm-unknown, but it is
certainly possible to extend it to other targets like GOOS=js and
wasip2.

src/runtime/runtime_wasm_unknown.go

@@ -2,7 +2,8 @@
 
 package runtime
 
-import "unsafe"
+// TODO: this is essentially reactor mode wasm. So we might want to support
+// -buildmode=c-shared (and default to it).
 
 type timeUnit int64
 
@@ -11,14 +12,6 @@ type timeUnit int64
 //export __wasm_call_ctors
 func __wasm_call_ctors()
 
-//export _initialize
-func _initialize() {
-	// These need to be initialized early so that the heap can be initialized.
-	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
-	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
-	initAll()
-}
-
 func init() {
 	__wasm_call_ctors()
 }
@@ -40,3 +33,6 @@ func sleepTicks(d timeUnit) {
 func ticks() timeUnit {
 	return timeUnit(0)
 }
+
+func beforeExit() {
+}