[CMake] Support passing arguments to build tool for external projects

Add CMake variable LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS to allow
arguments to be passed to the native tool used in CMake --build
invocations for external projects.

Can be used to pass extra arguments for enhanced versions of build
tools, e.g. distributed build options.

Differential Revision: https://reviews.llvm.org/D115815