cmd/link: vmsize of __DWARF should not be zero on darwin

[hirochachacha]

Please answer these questions before submitting your issue. Thanks!

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

Re-enable plugin mode for darwin/amd64.

diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 9a1204866e..705509de27 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -358,6 +358,8 @@ func BuildModeInit() {
 			switch platform {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386", "linux/s390x",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
+			case "darwin/amd64":
+				// cfg.BuildLdflags = append(cfg.BuildLdflags, "-w")
 			default:
 				base.Fatalf("-buildmode=plugin not supported on %s\n", platform)
 			}

Then try the trivial example in https://golang.org/pkg/plugin/

What did you expect to see?

success.

What did you see instead?

panic: plugin.Open: dlopen(/Users/hiro/plugin.so, 10): no suitable image found.  Did find:
	/Users/hiro/plugin.so: malformed mach-o image: segment __DWARF has vmsize < filesize
	/Users/hiro/plugin.so: malformed mach-o image: segment __DWARF has vmsize < filesize

goroutine 1 [running]:
main.main()
	/Users/hiro/p.go:8 +0x3a9

Does this issue reproduce with the latest release (go1.9)?

System details

go version devel +b046878018 Sat Aug 26 15:55:04 2017 +0000 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hiro/.go"
GORACE=""
GOROOT="/Users/hiro/go"
GOTOOLDIR="/Users/hiro/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wq/dwn8hs0x7njbzty9f68y61700000gn/T/go-build397205854=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version devel +b046878018 Sat Aug 26 15:55:04 2017 +0000 darwin/amd64
GOROOT/bin/go tool compile -V: compile version devel +b046878018 Sat Aug 26 15:55:04 2017 +0000
uname -v: Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; root:xnu-3789.70.16~2/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.12.6
BuildVersion:	16G29
lldb --version: lldb-370.0.42
  Swift-3.1
gdb --version: GNU gdb (GDB) 8.0

I'm going to skip DWARF generation now.

[gopherbot]

Change https://golang.org/cl/59372 mentions this issue: cmd/go: re-enable buildmode=plugin on darwin/amd64

[gopherbot]

Change https://golang.org/cl/182959 mentions this issue: cmd/go: remove -w workaround for buildmode=plugin on Darwin