compiler: add support for reading embedcfg files

This is the code that parses an embedcfg file, which is a JSON file
created by the go command when it sees go:embed directives.  This code
is not yet called, and does not yet do anything.  It's being sent as a
separate CL to isolate just the JSON parsing code.

	* Make-lang.in (GO_OBJS): Add go/embed.o.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/281532