[libc++] Allow running the bootstrapping CI build on Darwin

[ldionne]

Summary

Instead of hard-coding the target for our CI nodes, use the default
compiler triple. Also, allow building compiler-rt for the single
specified triple in case we're running on Darwin (otherwise, the
bootstrapping build complains).

[jloser]

@ldionne Thanks for this! I'll give it a try soon.

libcxx/utils/ci/run-buildbot

bootstrapping-build)
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/llvm" \
           -B "${BUILD_DIR}" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
           -DCMAKE_BUILD_TYPE=RelWithDebInfo \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang" \
           -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
-          -DLLVM_RUNTIME_TARGETS="x86_64-unknown-linux-gnu" \
+          -DLLVM_RUNTIME_TARGETS="$(c++ --print-target-triple)" \
+          -DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
           -DLLVM_ENABLE_ASSERTIONS=ON \
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-modules.cmake" \
           -DLIBCXX_TEST_CONFIG="llvm-libc++-shared.cfg.in" \
           -DLIBUNWIND_TEST_CONFIG="llvm-libunwind-shared.cfg.in"
 
     echo "+++ Running the libc++ and libc++abi tests"
     ${NINJA} -C "${BUILD_DIR}" check-runtimes