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

pkgs/build-support/cc-wrapper/default.nix

@@ -331,11 +331,18 @@ stdenv.mkDerivation {
     # compile, because it uses "#include_next <limits.h>" to find the
     # limits.h file in ../includes-fixed. To remedy the problem,
     # another -idirafter is necessary to add that directory again.
+    #
+    # We use --sysroot=/nix/store/does/not/exist to drop embedded default
+    # path to glibc headers gcc was built against. Without it -idirafter
+    # only appends to the list and outdated glibc headers end up being
+    # used. 'cc-cflags-before' is used to allow user's --sysroot= option
+    # to override our default.
     + optionalString (libc != null) (''
       touch "$out/nix-support/libc-cflags"
       touch "$out/nix-support/libc-ldflags"
       echo "-B${libc_lib}${libc.libdir or "/lib/"}" >> $out/nix-support/libc-crt1-cflags
     '' + optionalString (!(cc.langD or false)) ''
+      echo "--sysroot=/nix/store/does/not/exist" >> $out/nix-support/cc-cflags-before
       echo "-idirafter ${libc_dev}${libc.incdir or "/include"}" >> $out/nix-support/libc-cflags
     '' + optionalString (isGNU && (!(cc.langD or false))) ''
       for dir in "${cc}"/lib/gcc/*/*/include-fixed; do