libc/time: Fix `mktime()` wrong behavior when time can not be represented #13

Job	Run time
cache-maker	5s
minsize-zephyr (./.github/do-zephyr)	37m 47s
release-zephyr (./.github/do-zephyr)	38m 31s
minsize-zephyr (-Dio-float-exact=false -Dio-long-long=true -Dio-percent-b=true -Dio-long-double=t...	35m 36s
release-zephyr (-Dio-float-exact=false -Dio-long-long=true -Dio-percent-b=true -Dio-long-double=t...	37m 53s
minsize-zephyr (-Dformat-default=integer -Dfreestanding=true -Dposix-io=false -Dnewlib-obsolete-m...	36m 32s
release-zephyr (-Dformat-default=integer -Dfreestanding=true -Dposix-io=false -Dnewlib-obsolete-m...	38m 12s
minsize-zephyr (-Dtinystdio=false, ./.github/do-zephyr)	21m 33s
release-zephyr (-Dtinystdio=false, ./.github/do-zephyr)	23m 14s
minsize-zephyr (-Dtinystdio=false -Dnewlib-io-float=true -Dio-long-long=true -Dio-long-double=tru...	12m 59s
release-zephyr (-Dtinystdio=false -Dnewlib-io-float=true -Dio-long-long=true -Dio-long-double=tru...	12m 42s
	4h 55m 4s

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