[symbolizer] Change libcxx paths in buildscript.

[hctim]

Summary

D107799 changed the paths from lib/libcxx(abi)?.a to
lib/<triple>/libcxx(abi)?.a. The build script needs to know to pick up
the files from the triple subfolder instead.

See https://lab.llvm.org/buildbot/#/builders/37/builds/6764 for buildbot log failure.

[MaskRay]

Thanks!

compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh

 
 echo "Compiling..."
 SYMBOLIZER_FLAGS="$LLVM_FLAGS -I${LLVM_SRC}/include -I${LLVM_BUILD}/include -std=c++14"
 $CXX $SYMBOLIZER_FLAGS ${SRC_DIR}/sanitizer_symbolize.cpp ${SRC_DIR}/sanitizer_wrappers.cpp -c
 $AR rc symbolizer.a sanitizer_symbolize.o sanitizer_wrappers.o
 
 SYMBOLIZER_API_LIST=__sanitizer_symbolize_code,__sanitizer_symbolize_data,__sanitizer_symbolize_flush,__sanitizer_symbolize_demangle
 
+LIBCXX_ARCHIVE_DIR=$(dirname $(find $LIBCXX_BUILD -name libc++.a | head -n1))
+
 # Merge all the object files together and copy the resulting library back.
-$SCRIPT_DIR/ar_to_bc.sh $LIBCXX_BUILD/lib/libc++.a \
-                        $LIBCXX_BUILD/lib/libc++abi.a \
+$SCRIPT_DIR/ar_to_bc.sh $LIBCXX_ARCHIVE_DIR/libc++.a \
+                        $LIBCXX_ARCHIVE_DIR/libc++abi.a \
                         $LLVM_BUILD/lib/libLLVMSymbolize.a \
                         $LLVM_BUILD/lib/libLLVMObject.a \
                         $LLVM_BUILD/lib/libLLVMBinaryFormat.a \
                         $LLVM_BUILD/lib/libLLVMDebugInfoDWARF.a \
                         $LLVM_BUILD/lib/libLLVMSupport.a \
                         $LLVM_BUILD/lib/libLLVMDebugInfoPDB.a \
                         $LLVM_BUILD/lib/libLLVMDebugInfoMSF.a \
                         $LLVM_BUILD/lib/libLLVMDebugInfoCodeView.a \