Don't modify the host compiler's sysroot

[alexcrichton]

This commit updates the building of the wasi-sdk sysroot to leverage the -resource-dir argument from Clang to avoid modifying the host compiler's sysroot with compiler-rt things. This should help improve the experience of building a standalone sysroot with whatever host Clang is on the system.

Closes #444

[abrown]

cc: @whitequark, does this help towards resolving the discussion you were having over in #443 (comment)? I'm thinking of merging this but wanted to hear whether you had any additional feedback here.

[whitequark]

LGTM

[yamt]

have you ever tested if this change actually "avoid modifying the host compiler's sysroot with compiler-rt things"?
i don't think it works w/o WebAssembly/wasi-libc#517.
i suspect we need a bit more CI coverage.

[alexcrichton]

Yes I tested this doesn't modify the host compiler's sysroot. No there is no test of using the host toolchain in CI. Yes it would be great to add tests.

[yamt]

Yes I tested this doesn't modify the host compiler's sysroot.

did your "host compiler" already happen to have wasi compiler-rt?
otherwise, i don't understand why you didn't need WebAssembly/wasi-libc#517.
for me, gmake tried to create the directory for wasi compiler-rt in the host compiler's resource dir.

[alexcrichton]

On Ubuntu 24.04 I ran:

$ cmake -G Ninja -B build -S . -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_NM=llvm-nm-18
$ ninja -C build dist

and everything compiled ok. No the host compiler does not have a compiler-rt for wasm.

[yamt]

On Ubuntu 24.04 I ran:

$ cmake -G Ninja -B build -S . -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_NM=llvm-nm-18
$ ninja -C build dist

and everything compiled ok. No the host compiler does not have a compiler-rt for wasm.

where did clang/clang++/llvm-nm-18 come from? ubuntu package?

which compiler-rt does it link for wasi-libc without WebAssembly/wasi-libc#517?

[alexcrichton]

The clang package owns clang/clang++ and llvm-18 owns llvm-nm. Yes, they're from ubuntu packages.

As to what links compiler-rt is linked, I'm not sure.