cmd/compile: add basic wasmexport support

This CL adds a compiler directive go:wasmexport, which applies to
a Go function and makes it an exported function of the Wasm module
being built, so it can be called directly from the host. As
proposed in #65199, parameter and result types are limited to
32-bit and 64-bit integers and floats, and there can be at most
one result.

As the Go and Wasm calling conventions are different, for a
wasmexport function we generate a wrapper function does the ABI
conversion at compile time.

Currently this CL only adds basic support. In particular,
- it only supports executable mode, i.e. the Go wasm module calls
  into the host via wasmimport, which then calls back to Go via
  wasmexport. Library (c-shared) mode is not implemented yet.
- only supports wasip1, not js.
- if the exported function unwinds stacks (goroutine switch, stack
growth, etc.), it probably doesn't work.

TODO: support stack unwinding, c-shared mode, js.

For #65199.

Change-Id: Id1777c2d44f7d51942c1caed3173c0a82f120cc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/603055
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Randy Reddig <randy.reddig@fastly.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ir/func.go

@@ -142,6 +142,9 @@ type Func struct {
 	// WasmImport is used by the //go:wasmimport directive to store info about
 	// a WebAssembly function import.
 	WasmImport *WasmImport
+	// WasmExport is used by the //go:wasmexport directive to store info about
+	// a WebAssembly function import.
+	WasmExport *WasmExport
 }
 
 // WasmImport stores metadata associated with the //go:wasmimport pragma.
@@ -150,6 +153,11 @@ type WasmImport struct {
 	Name   string
 }
 
+// WasmExport stores metadata associated with the //go:wasmexport pragma.
+type WasmExport struct {
+	Name string
+}
+
 // NewFunc returns a new Func with the given name and type.
 //
 // fpos is the position of the "func" token, and npos is the position