added basic syscall handler that allows a task to yield, sleep and de… #410

Job	Run time
Build each header individually	53s
build (atsam3x8e)	21s
build (atsam4s2b)	20s
build (lpc802)	19s
build (lpc1752)	13s
build (lpc1788)	20s
build (lpc1754)	13s
build (max32625)	17s
build (lpc55s66)	16s
build (lpc1756)	12s
build (lpc1759)	20s
build (max32660)	13s
build (mb9bf566k)	13s
build (stm32h723)	19s
build (stm32f103)	18s
build (tmpm373)	19s
build (rp2350)	16s
build (stm32f407)	13s
	5m 35s

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