The error message seems to answer the question?

the error message shows "-buildmode=c-shared" not supported on linux/386, but "-buildmode=c-archive"
also not support on linux/386?

Over to @ianlancetaylor to clarify and/or fix the error message.

You need to tell us more. What files are in the directory in which you run the command? Are there any buildable Go source files there?

yes,there is Go sources file ,if i just use "go build -buildmode=c-archive" without "GOARCH=386",it can build an archive file that can be linked into a C program,my Go version is go1.5rc1

I can not recreate the problem. Can you show a small example with the exact .go files you are using?

I do see a completely different problems. When setting GOARCH with -buildmode=c-archive, you may need to add a go build argument
-ldflags=-linkmode=external

this is mytest.go
package main // import "mytest"

import _ "mytest/p"

func main() {} // ignored

this is p.go
package p //import "mytest/p"

import "C"

//export Foo
func Foo() int32 { return 42 }

use "GOARCH=386 go build -ldflags=-linkmode=external -buildmode=c-archive" failed

Ah. By default, cross-compilers do not enable cgo, so the go tool is ignoring p/p.go since it needs cgo. You need to say

CGO_ENABLED=1 GOARCH=386 go build -buildmode=c-archive

oh,thank you