[xla:cpu] Make HostKernel::Launch process all work in the main thread if thread_pool_ is nullptr.
[xla:cpu] Make HostKernel::Launch process all work in the main thread if thread_pool_ is nullptr.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/13047 from shawnwang18:shawnw/command_buffer_add_cublas_lt ee872b54d6301f2dfb1168b57dcd5b6e8a0c4561
