build

Refactor `gemma/common.cc` to improve readability and safety #3112

Job	Run time
bazel	5m 42s
macos-latest (make) Release	48s
ubuntu-20.04 (make) Release	1m 18s
ubuntu-latest (make) Release	1m 9s
windows-latest (windows) Release	16m 12s
	25m 9s

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