What version of Go are you using (go version)?

$ go version
go version go1.11.2 linux/amd64

But also tested with devel:

$ go version
go version devel +be09bdf589 Tue Dec 4 23:01:00 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/user/go"
GOPROXY=""
GORACE=""
GOROOT="/home/user/Google/go"
GOTMPDIR=""
GOTOOLDIR="/home/user/Google/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="clang-8"
CXX="clang++-8"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build949670883=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ cat wasm_log.go
package main

import (
        _ "unsafe" // for go:linkname
)

//go:linkname wasm_log wasm_log
func wasm_log(s string)

func main() {
        wasm_log("Hello, world!")
}

Note: the wasm_log function is provided by the host environment and is expected to be resolved at runtime, when the WASM module is instantiated.

What did you expect to see?

Compiled WASM module that communicates with the host environment.

For the comparison, it builds successfully with TinyGo, and the resulting WASM module works as expected:

$ tinygo build -target=wasm -no-debug -o wasm_log.wasm wasm_log.go

$ wavm-disas wasm_log.wasm wasm_log.wat
$ cat wasm_log.wat
(module
  (type $0 (func (result i32)))
  (type $1 (func (param i32 i32)))
  (type $2 (func))
  (import "env" "io_get_stdout" (func $io_get_stdout (result i32)))
  (import "env" "wasm_log" (func $wasm_log (param i32 i32)))
  (export "memory" (memory $4))
  (export "__heap_base" (global $6))
  (export "__data_end" (global $7))
  (export "_start" (func $_start))
  (export "cwa_main" (func $cwa_main))
  (memory $4  2)
  (table $3  1 1 anyfunc)
  (global $5  (mut i32) (i32.const 66576))
  (global $6  i32 (i32.const 66576))
  (global $7  i32 (i32.const 1037))
  (data $4 (i32.const 1024)
    "Hello, world!")

  (func $__wasm_call_ctors (type $2)
    )

  (func $_start (type $2)
    call $io_get_stdout
    drop
    )

  (func $cwa_main (type $2)
    call $io_get_stdout
    drop
    i32.const 1024
    i32.const 13
    call $wasm_log
    ))

What did you see instead?

With go1.11.2:

$ GOOS=js GOARCH=wasm go build -o wasm_log.wasm wasm_log.go
# command-line-arguments
./wasm_log.go:8:6: missing function body

With devel (thanks to https://golang.org/cl/151318):

$ GOOS=js GOARCH=wasm go build -o wasm_log.wasm wasm_log.go
# command-line-arguments
main.main: relocation target wasm_log not defined