Describe the bug

lib64 directories are added to linker rpath flags for cross-compilation to platforms that don't need them.

Steps To Reproduce

Steps to reproduce the behavior:

1. nix build -L github:eliasnaur/nix-lib64-rpath on a x86_64-linux system

output:

...
demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf> CC armv6l-unknown-linux-gnueabihf-gcc
demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf> NIX_LDFLAGS: -rpath /nix/store/lskdn4fvvjlz53dxkbkgb1x2dy4zkf7k-demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf/lib64 -rpath /nix/store/lskdn4fvvjlz53dxkbkgb1x2dy4zkf7k-demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf/lib
...

Note the lib64 directory added to NIX_LDFLAGS.

See full build log at https://github.com/eliasnaur/nix-lib64-rpath/actions/runs/4428890865/jobs/7768605092.

Running the same command on a aarch64-darwin configuration (with a aarch64-linux builder configured):

% nix build -L
...
demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf> CC armv6l-unknown-linux-gnueabihf-gcc
demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf> NIX_LDFLAGS: -rpath /nix/store/sw8ymyqpavw0v4hl01q0xb9c9wr50v9z-demonstrate-lib64-rpath-armv6l-unknown-linux-gnueabihf/lib
...

Note that NIX_LDFLAGS doesn't contain a lib64 directory.

Expected behavior

NIX_LDFLAGS should not contain lib64 directories when cross-compiling for non-x86_64-linux platforms.

Additional context

Adding lib64 to NIX_LDFLAGS is generally benign, except that I'm trying to achieve bit-for-bit reproducible builds. The extra lib64 entries end up in slightly larger ELF sections that can't be saved by careful use of patchelf.