[CMake] Support passing arguments to build tool (bootstrap).

For bootstrap builds (CLANG_ENABLE_BOOTSTRAP=ON) allow
arguments to be passed to the native tool used in CMake
for the stage2 step.

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

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D131665