llvmPackages_16.libunwind: fix aarch64-android build

[alexfmpe]

Description of changes

@axelkar @reckenrode

Ran into some failures when trying to build pkgsCross.aarch64-android.hello or anything really.
I don't know the repercussions of any of this, mostly took things from comments in #269077

Should this target staging or so even being android specific?

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[alexfmpe]

Seems to also fix the non-prebuild version of #147084

[paparodeo]

I don't know the repercussions of any of this, mostly took things from comments in #269077

i think a better description on why this is needed and what the repercussions are needed before this gets committed.

[paparodeo]

this seems out of place -- most / all options for the llvm toolchains are selected via cmake defines. why is this one different, and why is it ok for it to be unconditional. there needs to be a comment providing why this is required.

[RossComputerGuy]

Yeah, I think a comment explaining this would be great. I've been trying to use nixpkgs-review but on every system I try it crashes heh.

[axelkar]

Buiding hello works, but it doesn't run on an Android device.

$ file ./result/bin/hello
./result/bin/hello: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /system/bin/linker64, not stripped

# on Android device
/data/local/tmp $ ./hello
CANNOT LINK EXECUTABLE "./hello": library "libunwind.so.1" not found: needed by main executable

Edit: Building .\#pkgsCross.aarch64-android-prebuilt.hello works and runs:

$ file result/bin/hello 
result/bin/hello: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /system/bin/linker64, not stripped

# on Android device
/data/local/tmp $ ./hello
Hello, world!