cmd/link: doesn't propagate -I flags to the external linker

[benjaminp]

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

1.9.2

Does this issue reproduce with the latest release?

yes

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

linux_amd64

The problem

When link decides to use an external linker, it turns the -r flags it received into -rpath flags for the external linker. However, it doesn't pass the -I flag through to the external linker:

$ cat main.go 
package main
func main() {}
$ $GOROOT/pkg/tool/linux_amd64/compile -shared -o main.o main.go
$ $GOROOT/pkg/tool/linux_amd64/link -I myrtld -r myrpath -buildmode pie -linkmode external -o main main.o
$ readelf -ld main | egrep interpreter\|RUNPATH\|PATH
      [Requesting program interpreter: myrtld]
 0x000000000000001d (RUNPATH)            Library runpath: [myrpath]
$ $GOROOT/pkg/tool/linux_amd64/link -I myrtld -r myrpath -buildmode pie -linkmode external -o main main.o
$ readelf -ld main | egrep interpreter\|RUNPATH\|PATH
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
 0x000000000000000f (RPATH)              Library rpath: [myrpath]

Also, --enable-new-dtags should probably be passed to external GNU ld-compatible linkers for better fidelity with the internal ELF linker.