In particular, using -ldflags with -
In particular, a command like:
garble -literals build -ldflags='-X "main.foo=foo bar"'
would fail, because we would try to use "\"main" as the package name for
the -X qualified name, with the leading quote character.
This is because we used strings.Split(ldflags, " ").
Instead, use the same quoted.Split that cmd/go uses,
copied over thanks to x/tools/cmd/bundle and go:generate.
Updates #492.
Daniel Martí