[scudo] Try to fix standalone build on armv7

[rovka]

Summary

When linking scudo standalone on armv7, it can't find symbols related to
unwinding (e.g. __aeabi_unwind_cpp_pr0). This is probably because it
is passing --unwindlib=none. This patch hacks around the issue by adding
COMPILER_RT_UNWINDER_LINK_LIBS if we're using the LLVM unwinder.

I don't know anything about scudo, so I'm not sure what the original
intention was.

This is a release blocker.
See also https://github.com/llvm/llvm-project/issues/56900

[phosek]

This looks like a correct fix to me, thank you for looking into it.

compiler-rt/lib/scudo/standalone/CMakeLists.txt

138–140

You should be able to set this unconditionally since COMPILER_RT_UNWINDER_LINK_LIBS is already set (or unset) based on the content of COMPILER_RT_USE_LLVM_UNWINDER (in the future it might also expand to -lgcc on other platforms).

[rovka]

compiler-rt/lib/scudo/standalone/CMakeLists.txt

138–140

Makes sense, thanks for the quick review!