cmd/compile, cmd/link: encode cgo directives using JSON

The standard library has plenty of polished encoder/decoder
implementations. No need for another ad-hoc one.

I considered using encoding/gob instead, but these strings go into the
package data part of the object file, so it's important they don't
contain "\n$$\n". Package json escapes newlines in strings, so it's
safe to use here.

Change-Id: I998655524ccee7365c2c8e9a843e6975e95a3e62
Reviewed-on: https://go-review.googlesource.com/106463


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>