[compiler-rt] Relax CHECK line in reaches_function.c

In my build environment, the output is a relative path:
```
test/dfsan/reaches_function.c:34 add.dfsan
test/dfsan/reaches_function.c:56 main
```

This fixes check-dfsan in my configuration:
```
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -G Ninja
-DCMAKE_C_COMPILER=$HOME/output/upstream-llvm/bin/clang
-DCMAKE_CXX_COMPILER=$HOME/output/upstream-llvm/bin/clang++
-DCOMPILER_RT_INCLUDE_TESTS=ON
-DLLVM_EXTERNAL_LIT=$HOME/build/upstream-llvm-project-build/bin/llvm-lit
-DLLVM_CMAKE_DIR=$HOME/output/upstream-llvm
-DCOMPILER_RT_DEBUG=OFF
-S $HOME/src/upstream-llvm-project/compiler-rt
-B $HOME/src/upstream-llvm-project/compiler-rt/cmake-build-all-sanitizers
```

Reviewers: browneee, vitalybuka

Reviewed By: vitalybuka

Pull Request: https://github.com/llvm/llvm-project/pull/83089