build-support/cc-wrapper: pass in non-existent --sysroot= to untangle from libc

I would like to add an extra `gcc` build step during linux bootstrap
(https://github.com/NixOS/nixpkgs/issues/208412). This makes it early
bootstrap compiler linked and targeted against `bootstrapTools` `glibc`
including it's headers.

Without this change `gcc`'s spec files always prefer `bootstrapTools` `glibc`
for header search path (passed in as --with-native-system-header-dir=). We'can't
override it with:

- `-I` option as it gets stacked before gcc-specific headers, we need to keep
  glibc headers after gcc as gcc cleans namespace up for C standard by using
  #include_next and by undefining system macros.
- `-idirafter` option as it gets appended after existing `glibc`-includes

This `--sysroot=/nix/store/does/not/exist` hack allows us to remove existing
`glibc` headers and add new ones with `-idirafter`.

We use `cc-cflags-before` instead of `libc-cflags` to allow user to define
their own `--sysroot=` (like `firefox` does).

To keep it working prerequisite cross-symlink in gcc.libs is required:
https://github.com/NixOS/nixpkgs/pull/209153

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