Add reallocarray(3).

Originally a BSD extension, now in glibc too. We've used it internally
for a while.

(cherry-pick of e4b13f7e.)

Bug: http://b/112163459
Test: ran tests
Change-Id: I813c3a62b13ddb91ba41e32a5a853d09207ea6bc
Merged-In: I813c3a62b13ddb91ba41e32a5a853d09207ea6bc