cmd/go: add test for gccgo name mangling crash

Updates #33871

Change-Id: I73b1513a89ad89126159ce03ee72b922cd01916c
Reviewed-on: https://go-review.googlesource.com/c/go/+/200837
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/gccgo_mangle.txt

@@ -0,0 +1,15 @@
+# Issue 33871.
+
+cd m/a.0
+go build
+
+-- m/go.mod --
+module m
+-- m/a.0/a.go --
+package a
+
+type T int
+
+func (t T) M() int {
+	return int(t)
+}