build: Speed up kconfig_gen_symbols.py

Use a ProcessPoolExecutor to parallelize the processing done for each
architecture. This cuts the execution time of this script approximately
in half (saving ~15 seconds), and somewhat reducing the overall initial
setup.

Test: build
Change-Id: Ic95476fdd248ee9a90c37b07b354d03ef6a39960
Reviewed-on: https://pigweed-review.googlesource.com/c/zephyr/zephyr-bazel/+/431873