[compiler-rt] Reapply [compiler-rt] Check for and use -lunwind when linking with -nodefaultlibs (PR #66584)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Previously this failed with the following error:
```
-- Looking for _Unwind_RaiseException in unwind
-- Looking for _Unwind_RaiseException in unwind - not found
CMake Error at /usr/share/cmake-3.25/Modules/CheckLibraryExists.cmake:71 (try_compile):
  Only libraries may be used as try_compile or try_run IMPORTED
  LINK_LIBRARIES.  Got unwind of type UTILITY.
Call Stack (most recent call first): 
  /b/sanitizer-aarch64-linux-fuzzer/build/llvm-project/compiler-rt/cmake/config-ix.cmake:66 (check_library_exists)
  /b/sanitizer-aarch64-linux-fuzzer/build/llvm-project/compiler-rt/CMakeLists.txt:281 (include)
```

I tried fixing it by wrapping it all in an `if (NOT TARGET unwind)` - which works, but feels rather ugly...

https://github.com/llvm/llvm-project/pull/66584