like this

$ cat hello.c                      
#include <stdio.h>
int main() { printf("hello sizeof(void*) = %zd\n", sizeof(void*)); }
$ vim hello.c                
$ armv7l-unknown-linux-gnueabihf-gcc -o armhello -lrt -lpthread hello.c
$ qemu-arm
qemu: no user program specified
$ qemu-arm ./armhello 
hello sizeof(void*) = 4

like this

$ cat hello.c
#include <stdio.h>
int main() { printf("hello sizeof(void*) = %zd\n", sizeof(void*)); }
$ armv7l-unknown-linux-gnueabihf-gcc -o armhello -lrt -lpthread hello.c
$ qemu-arm ./armhello
hello sizeof(void*) = 4

So I simply tried nix-shell -p pkgsCross.armv7l-hf-multiplatform.stdenv.cc, if that is what you meant and got these errors:

/nix/store/viscj6hs10c5as7h53cxxw59yvgk505z-armv7l-unknown-linux-gnueabihf-binutils-2.43.1/bin/armv7l-unknown-linux-gnueabihf-ld: cannot find -lrt: No such file or directory
/nix/store/viscj6hs10c5as7h53cxxw59yvgk505z-armv7l-unknown-linux-gnueabihf-binutils-2.43.1/bin/armv7l-unknown-linux-gnueabihf-ld: cannot find -lpthread: No such file or direc
tory
/nix/store/viscj6hs10c5as7h53cxxw59yvgk505z-armv7l-unknown-linux-gnueabihf-binutils-2.43.1/bin/armv7l-unknown-linux-gnueabihf-ld: cannot find -lc: No such file or directory

So this turned out to work:

# flake.nix
{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  };
  outputs = {nixpkgs, ...}: let
    system = "x86_64-linux";
    pkgs = import nixpkgs {
      inherit system;
      crossSystem = {
        config = "armv7l-linux-gnueabihf";
      };
    };
  in {
    devShells.${system}.default = import ./nix/shell.nix {inherit pkgs;};
  };
}

and

# nix/shell.nix
{pkgs, ...}:
pkgs.mkShell {
  buildInputs = [
    pkgs.glibc
    pkgs.glibc.static
  ];

  makeFlags = [
    "CC=${pkgs.stdenv.cc.targetPrefix}cc"
    "CXX=${pkgs.stdenv.cc.targetPrefix}g++"
  ];
}

with pkgs.glibc.static being the thing that made the difference

Trying to fix pkgsLLVM.linux to work correctly, got this odd issue:

linux> make -f /build/linux-6.6.87/tools/build/Makefile.build dir=. obj=fixdep
linux> /nix/store/rpxlridr3yn6mbmd34r0bya9qv91b79s-gcc-wrapper-14.2.1.20250322/bin/cc   -o /build/linux-6.6.87/build/tools/bpf/resolve_btfids/fixdep /build/linux-6.6.87/build/tools/bpf/resolve_btfids/fixdep-in.o
linux> mkdir -p /build/linux-6.6.87/build/tools/bpf/resolve_btfids//libsubcmd
linux> make -C /build/linux-6.6.87/tools/lib/subcmd/ OUTPUT=/build/linux-6.6.87/build/tools/bpf/resolve_btfids/libsubcmd/ \
linux>             DESTDIR=/build/linux-6.6.87/build/tools/bpf/resolve_btfids/libsubcmd/ AR="/nix/store/0w9bd0ljiyj2crmywkm0mvm4nwsswgb0-binutils-wrapper-2.44/bin/ar" CC="/nix/store/rpxlridr3yn6mbmd34r0bya9qv91b79s-gcc-wrapper-14.2.1.20250322/bin/cc" LD="/nix/store/0w9bd0ljiyj2crmywkm0mvm4nwsswgb0-binutils-wrapper-2.44/bin/ld" ARCH="arm64" CROSS_COMPILE="" EXTRA_CFLAGS="" prefix= subdir= \
linux>             /build/linux-6.6.87/build/tools/bpf/resolve_btfids/libsubcmd/libsubcmd.a install_headers
linux> exec-cmd.c:2:10: fatal error: linux/compiler.h: No such file or directory
linux>     2 | #include <linux/compiler.h>
linux>       |          ^~~~~~~~~~~~~~~~~~
linux> compilation terminated.
linux> make[6]: *** [/build/linux-6.6.87/tools/build/Makefile.build:98: /build/linux-6.6.87/build/tools/bpf/resolve_btfids/libsubcmd/exec-cmd.o] Error 1
linux> make[5]: *** [Makefile:80: /build/linux-6.6.87/build/tools/bpf/resolve_btfids/libsubcmd/libsubcmd-in.o] Error 2
linux> make[4]: *** [Makefile:58: /build/linux-6.6.87/build/tools/bpf/resolve_btfids//libsubcmd/libsubcmd.a] Error 2
linux> make[3]: *** [Makefile:76: bpf/resolve_btfids] Error 2
linux> make[2]: *** [/build/linux-6.6.87/Makefile:1362: tools/bpf/resolve_btfids] Error 2
linux> make[1]: *** [/build/linux-6.6.87/Makefile:234: __sub-make] Error 2
linux> make[1]: Leaving directory '/build/linux-6.6.87/build'
linux> make: *** [Makefile:234: __sub-make] Error 2