glibc: symlink libpthread.so -> libpthread.so.0 (same for -lrt) for backwards compatibility

Linking via `-lpthread` (or `-pthread`) is not needed anymore since
`glibc-2.34` since all the functionality is part of `libc.so.6` and
`libpthread.so.6` only exists for backwards-compatibility.

However, e.g. `gcc` (`libgomp` to be precise) expects a `libpthread.so`
to link against, otherwise the configure script will fail. As already
stated in the glibc release-notes itself, it is to expect that a lot
more applications will have issues with this, so I decided to re-add
`libpthread.so` as well.

For `librt.so.1`, the same thing is needed to make sure that Perl still
compiles:

    /nix/store/d6y5r7m93x14bmgn2p75fannz39jz66f-binutils-2.35.1/bin/ld: cannot find -lrt
    collect2: error: ld returned 1 exit status
    make[1]: *** [Makefile:490: ../../lib/auto/Time/HiRes/HiRes.so] Error 1
    make[1]: Leaving directory '/build/perl-5.34.0/dist/Time-HiRes'

pkgs/development/libraries/glibc/default.nix

@@ -119,6 +119,14 @@ callPackage ./common.nix { inherit stdenv; } {
 
       # Get rid of more unnecessary stuff.
       rm -rf $out/var $bin/bin/sln
+
+      # Backwards-compatibility to fix e.g.
+      # "configure: error: Pthreads are required to build libgomp" during `gcc`-build
+      # because it's not actually needed anymore to link against `pthreads` since
+      # it's now part of `libc.so.6` itself, but the gcc build breaks if
+      # this doesn't work.
+      ln -sf $out/lib/libpthread.so.0 $out/lib/libpthread.so
+      ln -sf $out/lib/librt.so.1 $out/lib/librt.so
     ''
       # For some reason these aren't stripped otherwise and retain reference
       # to bootstrap-tools; on cross-arm this stripping would break objects.