cc-wrapper: change cflags order from cc/crt1/libc to crt1/libc/cc

In https://github.com/NixOS/nixpkgs/issues/158042 I noticed order
mismatch as a bootstrap build failure when building x86_64-linux
against glibc-2.35 in nixpkgs (bootstrap libs has glibc-2.27):

expand-response-params> ld: /nix/store/p4s4jf7aq6v6z9iazll1aiqwb34aqxq9-bootstrap-tools/lib/crt1.o: in function `_start':
expand-response-params> /build/glibc-2.27/csu/../sysdeps/x86_64/start.S:101: undefined reference to `__libc_csu_fini'
expand-response-params> ld: /build/glibc-2.27/csu/../sysdeps/x86_64/start.S:102: undefined reference to `__libc_csu_init'
expand-response-params> collect2: error: ld returned 1 exit status

Here crt1.o from glibc-2.27 links against libc.so.6 from glibc-2.35.

This happens because ordering of `-L` (influences `libc.so` lookup) and
`-B` (influences `crt1.o` lookup) flags differs:

expand-response-params> -B/...-bootstrap-tools/lib
expand-response-params> -B/...-glibc-2.35/lib/
...
expand-response-params> -L/...-glibc-2.35/lib
expand-response-params> -L/...-bootstrap-tools/lib

The change makes consistent ordering of `-L`/`-B` and allows getting to
stage4 for `glibc-2.35` target.

pkgs/build-support/cc-wrapper/add-flags.sh

···
 # Export and assign separately in order that a failing $(..) will fail
 # the script.
 
+# Currently bootstrap-tools does not split glibc, and gcc files into
+# separate directories. As a workaround we want resulting cflags to be
+# ordered as: crt1-cflags libc-cflags cc-cflags. Otherwise we mix crt/libc.so
+# from different libc as seen in
+#   https://github.com/NixOS/nixpkgs/issues/158042
+#
+# Note that below has reverse ordering as we prepend flags one-by-one.
+# Once bootstrap-tools is split into different directories we can stop
+# relying on flag ordering below.
+
+if [ -e @out@/nix-support/cc-cflags ]; then
+    NIX_CFLAGS_COMPILE_@suffixSalt@="$(< @out@/nix-support/cc-cflags) $NIX_CFLAGS_COMPILE_@suffixSalt@"
+fi
+
 if [[ "$cInclude" = 1 ]] && [ -e @out@/nix-support/libc-cflags ]; then
     NIX_CFLAGS_COMPILE_@suffixSalt@="$(< @out@/nix-support/libc-cflags) $NIX_CFLAGS_COMPILE_@suffixSalt@"
 fi
···
 
 if [ -e @out@/nix-support/libcxx-ldflags ]; then
     NIX_CXXSTDLIB_LINK_@suffixSalt@+=" $(< @out@/nix-support/libcxx-ldflags)"
-fi
-
-if [ -e @out@/nix-support/cc-cflags ]; then
-    NIX_CFLAGS_COMPILE_@suffixSalt@="$(< @out@/nix-support/cc-cflags) $NIX_CFLAGS_COMPILE_@suffixSalt@"
 fi
 
 if [ -e @out@/nix-support/gnat-cflags ]; then