wasm: do not allow undefined symbols

--allow-undefined can be a problem: it allows compiling code that will
fail when loaded. This change makes sure that if some symbols are
undefined, they are reported as an error by the linker.

Previously, people could get away with importing a function that was not
defined, like this:

    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

This was always unintended but mostly worked. With this change, it isn't
possible anymore. Now every function needs to be marked with //export
explicitly:

    //export add
    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

As before, functions will be placed in the `env` module with the name
set from the `//export` tag. This can be overridden with
`//go:import-module`:

    //go:import-module math
    //export add
    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

For the syscall/js package, I needed to give a list of symbols that are
undefined. This list is based on the JavaScript functions defined in
targets/wasm_exec.js.

compiler/symbol.go

@@ -163,18 +163,20 @@ func (c *compilerContext) getFunction(fn *ssa.Function) llvm.Value {
 	// External/exported functions may not retain pointer values.
 	// https://golang.org/cmd/cgo/#hdr-Passing_pointers
 	if info.exported {
-		// Set the wasm-import-module attribute if the function's module is set.
-		if info.module != "" {
-
+		if c.archFamily() == "wasm32" {
 			// We need to add the wasm-import-module and the wasm-import-name
-			wasmImportModuleAttr := c.ctx.CreateStringAttribute("wasm-import-module", info.module)
-			llvmFn.AddFunctionAttr(wasmImportModuleAttr)
-
-			// Add the Wasm Import Name, if we are a named wasm import
-			if info.importName != "" {
-				wasmImportNameAttr := c.ctx.CreateStringAttribute("wasm-import-name", info.importName)
-				llvmFn.AddFunctionAttr(wasmImportNameAttr)
+			// attributes.
+			module := info.module
+			if module == "" {
+				module = "env"
 			}
+			llvmFn.AddFunctionAttr(c.ctx.CreateStringAttribute("wasm-import-module", module))
+
+			name := info.importName
+			if name == "" {
+				name = info.linkName
+			}
+			llvmFn.AddFunctionAttr(c.ctx.CreateStringAttribute("wasm-import-name", name))
 		}
 		nocaptureKind := llvm.AttributeKindID("nocapture")
 		nocapture := c.ctx.CreateEnumAttribute(nocaptureKind, 0)

compiler/testdata/pragma.ll

@@ -62,7 +62,7 @@ declare void @main.undefinedFunctionNotInSection(i8*) #0
 
 attributes #0 = { "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #1 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="extern_func" }
+attributes #2 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="extern_func" "wasm-import-module"="env" "wasm-import-name"="extern_func" }
 attributes #3 = { inlinehint nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
 attributes #4 = { noinline nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" }
-attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" }
+attributes #5 = { nounwind "target-features"="+bulk-memory,+nontrapping-fptoint,+sign-ext" "wasm-export-name"="exportedFunctionInSection" "wasm-import-module"="env" "wasm-import-name"="exportedFunctionInSection" }