glibc: re-enable stripping in cross builds

[delroth]

Motivation for this change

References to the host toolchain are leaking through debug symbols in glibc, causing gnu cross-builds to always depend on the host toolchain. The decision to not strip was made in 2009 (c6b5264), and I suspect the reasons that justified it back then do not apply anymore in 2019.

Closure size before:
/nix/store/v5pxj0bgg627hic2khk4d43z6cjp5v7d-hello-2.10-armv7l-unknown-linux-gnueabihf 596.8M

After:
/nix/store/llp1ncmpar406rc2vhj7g5ix4yqwna3n-hello-2.10-armv7l-unknown-linux-gnueabihf 23.6M

Fixes #58501 (as far as I can tell). @illegalprime @Thra11 @matthewbauer

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[matthewbauer]

Also should fix #58981 and and #35251

[matthewbauer]

@GrahamcOfBorg build pkgsCross.aarch64-multiplatform.hello

[delroth]

I don't think it will fix #58981. That one will likely require #58606 to be merged as well to split libgcc_s.

[Ericson2314]

Yes please! Virtually any cross hack pre-2016 is probably obsoleted and we should try getting rid of it.