libc fortify: make socket.h and stdlib.h use diagnose_if

Since realpath no longer needs to be overloaded, we can restore the
upstream source to purity. We'll be able to do this with most of the
other functions when we pull a newer clang in.

Bug: 12231437
Test: m checkbuild on bionic internal master; CtsBionicTestCases show
no new failures.

Change-Id: I484221bba0b291273fece23d2be2f5f9fd713d2c