libaom: fix cross

[ghost]

Description of changes

Apparently the CMake variable for indicating the assembler has changed from AS_EXECUTABLE to CMAKE_ASM_COMPILER.

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/)
• 23.11 Release Notes (or backporting 23.05 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.

[NickCao]

@ofborg build pkgsCross.aarch64-multiplatform.libaom

[Artturin]

CI failed

CMake Error at build/cmake/aom_configure.cmake:192 (message):
  Unable to find assembler and optimizations are enabled.Searched for as.
  Install it, add it to your path, or set the assembler directly by adding
  -DAS_EXECUTABLE=<assembler path> to your CMake command line.To build
  without optimizations, add -DAOM_TARGET_CPU=generic to your cmake command
  line.
Call Stack (most recent call first):
  CMakeLists.txt:66 (include)

pkgsCross.aarch64-multiplatform.libaom builds on master

[uninsane]

pkgsCross.aarch64-multiplatform.libaom does not build from x86_64 for me on master (653e323). this patch fixes it.

[Artturin]

That's true, dunno what happened.