Locally provide crates that `libgrust` depends on

[tschwinge]

As of #2822 "Start reusing rustc's format_args parser", we have:

[...]
make[3]: Entering directory '[...]/build-gcc/gcc'
cargo build --manifest-path [...]/source-gcc/gcc/../libgrust/libformat_parser/Cargo.toml --release # FIXME: Not always release, right?
    Updating crates.io index
 Downloading crates ...
  Downloaded unicode-xid v0.2.4
  Downloaded libc v0.2.152
   Compiling libc v0.2.152
   Compiling unicode-xid v0.2.4
   Compiling generic_format_parser v0.1.0 ([...]/source-gcc/libgrust/libformat_parser/generic_format_parser)
   Compiling libformat_parser v0.1.0 ([...]/source-gcc/libgrust/libformat_parser)
    Finished release [optimized] target(s) in 2.39s
[...]

For different reasons, GCC needs to build without Internet access. Therefore, need to locally provide crates that libgrust depends on, either by importing into libgrust, or by new --with-[...] configure flags and instructions how to download/prepare them externally in a suitable format (source or binary).

[tschwinge]

I note:

$ cargo build --help
[...]
      --offline               Run without accessing the network
[...]