gcc/common: add disableGdbPlugin option

This commit adds an option `disableGdbPlugin` which controls whether
or not the plugin *for* GDB will be built.  This plugin contains a
copy of `gcc`.

The configure flag that this option controls is called
`--disable-libcc1`.  This flag name is slightly confusing: it is
used only by GDB (and apparently unmaintained), yet the flag name
does not mention GDB.  This is why the option name is different from
the configure flag name.

The primary motivation for this commit is to allow the following PR
(which is not yet merged) to pass `--disable-libcc1` when building
the final native (build=host=target) compiler as part of the stdenv
bootstrap:

  https://github.com/NixOS/nixpkgs/pull/209870

We need to `--disable-libcc1` in this scenario because gcc's build
machinery links `libcc1` against the `libstdc++` that is part of the
*compiler used to compile gcc*, rather than against the `libstdc++`
that is built *by* gcc.  In an FHS distribution this distinction is
not terribly important because dynamically linked libraries are
late-bound (ld.so resolution).  However in nixpkgs this causes a
reference back to the bootstrapFiles to leak all the way through to
the final stdenv.

More details can be found in the comment in
`pkgs/stdenv/linux/default.nix` of the PR linked above.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

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