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/link/internal/loader/loader.go

@@ -251,6 +251,8 @@ type Loader struct {
 	// CgoExports records cgo-exported symbols by SymName.
 	CgoExports map[string]Sym
 
+	WasmExports []Sym
+
 	flags uint32
 
 	strictDupMsgs int // number of strict-dup warning/errors, when FlagStrictDups is enabled
@@ -1627,6 +1629,10 @@ func (l *Loader) WasmImportSym(fnSymIdx Sym) Sym {
 	return l.aux1(fnSymIdx, goobj.AuxWasmImport)
 }
 
+func (l *Loader) WasmTypeSym(s Sym) Sym {
+	return l.aux1(s, goobj.AuxWasmType)
+}
+
 // SEHUnwindSym returns the auxiliary SEH unwind symbol associated with
 // a given function symbol.
 func (l *Loader) SEHUnwindSym(fnSymIdx Sym) Sym {
@@ -2213,6 +2219,9 @@ func (st *loadState) preloadSyms(r *oReader, kind int) {
 		if a := int32(osym.Align()); a != 0 && a > l.SymAlign(gi) {
 			l.SetSymAlign(gi, a)
 		}
+		if osym.WasmExport() {
+			l.WasmExports = append(l.WasmExports, gi)
+		}
 	}
 }