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.

compiler/compiler.go

@@ -44,6 +44,7 @@ type Config struct {
 	ABI             string
 	GOOS            string
 	GOARCH          string
+	BuildMode       string
 	CodeModel       string
 	RelocationModel string
 	SizeLevel       int
@@ -1384,6 +1385,11 @@ func (b *builder) createFunction() {
 		b.llvmFn.SetLinkage(llvm.InternalLinkage)
 		b.createFunction()
 	}
+
+	// Create wrapper function that can be called externally.
+	if b.info.wasmExport != "" {
+		b.createWasmExport()
+	}
 }
 
 // posser is an interface that's implemented by both ssa.Value and