cmd/ld: add -extld and -extldflags options

Permits specifying the linker to use, and trailing flags to
pass to that linker, when linking in external mode.  External
mode linking is used when building a package that uses cgo, as
described in the cgo docs.

Also document -linkmode and -tmpdir.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8225043