missing atomic_exchg_*() in the atomic.h API ?

while looking at the functions in atomic.h i noticed that
there seems to be no atomic_exchg_*() in the API, though this
is a supported function in most/all supported archiectures,
and a useful function. We do have atomic_readandclear() which
uses the same underlying CPU instruction.

Again, any objection if i add it ?

	cheers
	luigi