fix: ggml: fix vulkan-shaders-gen build

[sparkleholic]

The vulkan-shaders-gen target was not being built correctly in case of cross-compilation.
Other outputs need to be built for the cross compile target, but vulkan-shaders-gen needs to be built for the host.

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[sparkleholic]

@jeffbolznv , @0cc4m
This fix is ​​needed for cross-compilation.
vulkan-shaders-gen is needed at build time, so arch should be the same as host pc, but other binaries (libggml, libllama, etc.) are needed at runtime.
Therefore, CMAKE compilation variables that apply when building the llama.cpp project should not apply when building vulkan-shaders-gen in case of cross-compilation.

[0cc4m]

@sparkleholic @jeffbolznv I understand, I'm sorry that a few PRs are getting delayed because of me. I'm currently ill, but getting better and I should be able to catch up with all of them later this week.

Maybe you can resolve the merge conflict already?

[jeffbolznv]

No rush, get well!

I'm not an expert at cmake or cross compilation, so I don't think I should review the change.

[sparkleholic]

@0cc4m
master rebased to resolve conflicts.
I didn't mean to rush you. I sincerely hope you feel better soon. :)

[0cc4m]

I can confirm this still works as usual on my system, but I'm also not a CMake expert. Maybe @bandoti can take a look?

[bandoti]

I am happy to take a look. The one thing that comes to mind here after initial look through is being able to customize the compiler in this case. In particular, gcc and g++ are hardcoded.

It would be worthwhile to add a default toolchain file for the build and allow one to customize by passing -DGGML_SHADERS_GEN_CROSS_TOOLCHAIN=... (or similar)

This would allow one to use a desire toolchain—maybe clang, etc.

And a lot of those switches passed to ExternalProject_Add() can of course be moved to the toolchain file instead.

[sparkleholic]

@bandoti
Thank you for the valuable feedback.

I have updated the code to handle the GGML_SHADERS_GEN_TOOLCHAIN variable. When this variable is provided, the specified file is used. If it is not set, the code generates a temporary host toolchain file by detecting the host environment's compiler.

Please review the updated code once again.