cmd/link: don't pass -Wl,--dynamic-linker if -static

As with -rdynamic, clang will pass -Wl,--dynamic-linker to the linker
even when linking statically. When using lld this will produce a statically
linked executable with a dynamic interpreter, which will crash at runtime.
This CL changes the linker to drop -Wl,--dynamic-linker when using -static,
as it already does with -rdynamic.

This has become more important since CL 310349, which changes the linker
to always pass a -Wl,--dynamic-linker option if the Go linker is invoked
with a -I option.

Change-Id: I68ed431064f02c70018bc0547585e5b0ebd20a41
Reviewed-on: https://go-review.googlesource.com/c/go/+/314412


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>