Bump third_party/mbedtls/repo from `5bc604f` to `9870921` #2383

Job	Run time
Build on Darwin (clang, simulated)	51m 1s
Build on Linux (fake, gcc_release, clang, simulated)	54m 50s
Build on Linux (coverage)	13m 8s
Build on Linux (gcc_debug)	1h 6m 19s
Build on Linux (python_lib)	24m 13s
Build on Linux (python lighting-app)	6m 17s
	3h 35m 48s

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