[native_toolchain_c] Compile with `-Os` by default #918

Job	Run time
health / coverage	0s
health / changelog / run changelog health check	49s
health / breaking / run breaking health check	1m 1s
health / do-not-submit / run do-not-submit health check	44s
health / license / run license health check	43s
health / leaking / run leaking health check	50s
health / comment	5s
	4m 12s

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