Bump third_party/mbedtls/repo from `5bc604f` to `9870921` #405

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 0m 47s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 42s
Build on Linux (coverage)	14m 5s
Build on Linux (gcc_debug)	1h 1m 7s
Build on Linux (python_lib)	32m 51s
Build on Linux (python lighting-app)	6m 59s
	5h 54m 31s

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