test: enable bug385_32 test on amd64p32.

LGTM=dave
R=dave, rsc
CC=golang-codereviews
https://golang.org/cl/78110043

test/fixedbugs/bug385_32.go

-// +build 386 arm
+// +build 386 amd64p32 arm
 // errorcheck
 
 // Copyright 2011 The Go Authors.  All rights reserved.