In Linux ld.so itself has a default directory it loads libc from:

$ ld.so --help
...
Shared library search path:
  (libraries located via /nix/store/ibp4camsx1mlllwzh32yyqcq2r2xsy1a-glibc-2.37-8/etc/ld.so.cache)
  /nix/store/ibp4camsx1mlllwzh32yyqcq2r2xsy1a-glibc-2.37-8/lib (system search path)
  /nix/store/5cfwfnp4capzbjmkkrl7a6shvaghgj3h-xgcc-12.3.0-libgcc/lib (system search path)

For example linux does it via steps like https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/linux/default.nix#L412:

  (prevStage:
    stageFun prevStage {
    name = "bootstrap-stage2";
    # ...
    overrides = self: super: {
      inherit (prevStage)
        ccWrapperStdenv gettext
        gcc-unwrapped coreutils gnugrep
        perl gnum4 bison texinfo which;

Here perl and friends come from a previous instance of stdenv instead of being built normally.

In reply to @snektron:matrix.org
And it looks like starfive finally added pci support in uboot

it's in ld-wrapper: pkgs/build-support/bintools-wrapper/ld-wrapper.sh:

        for path in "$dir"/*; do
            file="${path##*/}"
            if [ "${libs[$file]:-}" ]; then
                rpaths["$dir"]=1
                extraAfter+=(-rpath "$dir")
                break
            fi
        done

I believe dynamic linker is set inf a slightly different way: via nix-support/ld-set-dynamic-linker file indirection. From pkgs/build-support/bintools-wrapper/add-flags.sh:

if [ -z "$NIX_DYNAMIC_LINKER_@suffixSalt@" ] && [ -e @out@/nix-support/ld-set-dynamic-linker ]; then
    NIX_DYNAMIC_LINKER_@suffixSalt@="$(< @out@/nix-support/dynamic-linker)"
fi

and later in ld-wrapper: pkgs/build-support/bintools-wrapper/ld-wrapper.sh:

extraBefore+=("-dynamic-linker" "$NIX_DYNAMIC_LINKER_@suffixSalt@")

so if I'm getting this, that means some substitution step isn't working correctly right?

/nix/store/aj621ihplrq2bjan90mj0j8jzd8yykai-pkg-config-wrapper-0.29.2/nix-support/setup-hook: line 61: export: `NIX_@wrapperName@_TARGET_HOST_@suffixSalt@=1': not a valid identifier

Yeah, normally substitution happens in pkgs/stdenv/generic/setup.sh: consumeEntire content < "$hook"; substituteAllStream content "file '$hook'" >> "${!outputDev}/nix-support/setup-hook". I think that means your environment does not define wrapperName variable when you build the derivation with a hook. Normally it is defined:

nix-repl> pkg-config.setupHooks
[ /home/slyfox/dev/git/nixpkgs/pkgs/build-support/setup-hooks/role.bash /home/slyfox/dev/git/nixpkgs/pkgs/build-support/pkg-config-wrapper/setup-hook.sh ]

nix-repl> pkg-config.wrapperName
"PKG_CONFIG_WRAPPER"