This fixes the build since 8f90e69
which made libcxxabi use llvm's libunwind by default.

…file data.

The new CLANG_PGO_TRAINING_DATA_SOURCE_DIR allows users to specify a CMake
project to use for generating the profile data.

For example, to use the llvm-test-suite to generate profile data you
would do:

$ cmake -G Ninja -B build -S llvm -C <path to source>/clang/cmake/caches/PGO.cmake \
        -DBOOTSTRAP_CLANG_PGO_TRAINING_DATA_SOURCE_DIR=<path to llvm-test-suite> \
        -DBOOTSTRAP_CLANG_PERF_TRAINING_DEPS=runtimes

Co-authored-by: Petr Hosek <[email protected]>