cmd/go: fix linux-amd64-clang builder

Fixes #10660

Fix the clang only builder by passing -extld down to the linker when needed.
The build passed on most hosts because gcc is almost always present. The bug
was verified by symlinking bin/false in place of gcc in my $PATH and running
the build.

Also, resolve a TODO and move the support logic into its own function.

Tested manually

    env CC=clang-3.5 ./all.bash	# linux/amd64
    env CC=gcc-4.8 ./all.bash	# linux/amd64
    ./all.bash			# linux/amd64
    ./all.bash			# darwin/amd64

Change-Id: I4e27a1119356e295500a0d19ad7a4ec14207bf10
Reviewed-on: https://go-review.googlesource.com/9526


Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>