cmd/link/internal/ld: if -v, display external linker output

cmd/link/internal/ld: if -v, display external linker output

It can be useful when debugging to be able to see what the external
linker is doing even when it succeeds.  In particular this permits
passing -v to the external linker to see precisely what it is doing.

Change-Id: Ifed441912d97bbebea20303fdb899e140b380215
Reviewed-on: https://go-review.googlesource.com/10363
Reviewed-by: Minux Ma <minux@golang.org>