cmd/compile: on Wasm and AIX, let deferred nil function panic at invocation

The Go spec requires

	If a deferred function value evaluates to nil, execution
	panics when the function is invoked, not when the "defer"
	statement is executed.

On Wasm and AIX, currently we actually emit a nil check at the
point of defer statement, which will make it panic too early.
This CL fixes this.

Also, on Wasm, now the nil function will be passed through
deferreturn to jmpdefer, which does an explicit nil check and
calls sigpanic if it is nil. This sigpanic, being called from
assembly, is ABI0. So change the assembler backend to also
handle sigpanic in ABI0.

Fixes #34926.
Updates #8047.

Change-Id: I28489a571cee36d2aef041f917b8cfdc31d557d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/201297
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/defernil.go

@@ -0,0 +1,33 @@
+// run
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that deferring a nil function causes a proper
+// panic when the deferred function is invoked (not
+// when the function is deferred).
+// See Issue #8047 and #34926.
+
+package main
+
+var x = 0
+
+func main() {
+	defer func() {
+		err := recover()
+		if err == nil {
+			panic("did not panic")
+		}
+		if x != 1 {
+			panic("FAIL")
+		}
+	}()
+	f()
+}
+
+func f() {
+	var nilf func()
+	defer nilf()
+	x = 1
+}